Thanks for being interested in such matters. I think that’s a good question. First of all, I also don’t want to debate or convince anybody, so please read all I’ll say as being my opinion, according to how I understand things at the moment. I also don’t know everything, and like you would like some clarification on some of these point.
I’m aware the suttas don’t use the term nimitta in the way it later was employed by the commentaries, but I’ll use this term regardless, because it is so common nowadays.
To answer the question, the jhanas are states which are very hard to describe, which makes me wonder to what extent people may be talking about the same thing with different words, disagreeing only on the surface. However, there is a very distinct shift from the nimitta stage into the first jhana, much more distinct than any perceptive change that happened prior. In first jhana the object is the pitisukha, no longer the lights or shapes (or sounds or whatever) of the nimittas. This doesn’t mean the jhana is a black darkness, it just means that all perceptions of color and of light/dark are abandoned. They are of no more relevance to the mind at that point.
To give some textual support for this, in MN128 the Buddha says he developed immeasurable lights and forms, but only afterwards developed the jhanas. AN8.120 & DN16 speak about eight liberations, where between the formless attainments and “perceiving forms” (which are the same as the kasinas, i.e. nimittas), there is “focused only on beauty”. I take this beauty to refer to the four jhanas. The amount of beauty (i.e. amount of pitisukha) can still vary from individual to individual, determining the strength of the jhana. But the focus isn’t on colors or shapes anymore.
Though it is a bit inferential, we can also derive this from the description of the jhanas themselves, which don’t mention form or lights but do mention piti and sukha. I see these discriptions—which are repeated all over the place with little to no change—as being very complete. The jhana similies, like at AN5.28 (links to my translation) also say: “You drenched, suffused, filled, and pervaded your experience with delight and bliss caused by the separation, so that there is no single part of your whole experience that is not pervaded with it.” Hence the object is just piti sukha, not lights. This is also indicated in other places, but let me leave it at that.
Rūpa’s fundamental meaning, from which all others were derived, is things seen, i.e. appearances/sights. When we speak of “the four elements” nowadays we might think of physical stuff, but Chandogya Upanishad 6.4.1 says the color red is a form of the fire element (rohitaṃ rūpaṃ tejasas), and the color white a form of the water element. The vedas have more such references like this that clarify that “form” isn’t physical matter as we think of it nowadays. In case of nimittas these forms are seen with the mind, although they can be almost indistinguishable from actual physical lights/colors, which is why I think the suttas sometimes call them “external forms”, like in the eight bases for mastering [samādhi]. They can definitely feel as if they are external to the meditator. When building a meditation center, some monks even made sure the meditation hall contained no windows, so the meditators don’t think nimittas are the sun shining in their eyes.
The eight bases are described in detail in MN77, DN16, AN8.65, and AN10.29 (so they’re actually more common than certain famous passages in the satipatthana suttas) and in the first two they precede the jhanas in order. These eight bases mention seeing (passati) forms, which are certain limited or measureless colors, dull or bright. However we interpret this, it is definitely not about feeling form “with the body”. The commentaries are a bit terse but call the external form here nimittas, the internal form being parts of the body, and I agree with that. I interpret the eight bases as follows:
There are these eight bases for mastering [samādhi]. What eight? Perceiving form internally [i.e. still feeling the body], someone sees forms externally [i.e. “sees” nimittas], which are limited in size, both shiny and dull [i.e. nimittas that have edges and are not bright all the time]. Mastering them, they perceive: ‘I know and see.’ This is the first basis for mastering.
Perceiving form internally, someone sees forms externally, limitless [i.e. nimittas which have no edges], both shiny and dull. Mastering them, they perceive: ‘I know and see.’ This is the second basis for mastering.
Not perceiving form internally [i.e. now no longer aware of the body, which is a deeper state of meditation], someone sees forms externally, which are limited in size, both shiny and dull. Mastering them, they perceive: ‘I know and see.’ This is the third basis for mastering.
Not perceiving form internally, someone sees forms externally, limitless, both shiny and dull. Mastering them, they perceive: ‘I know and see.’ This is the fourth basis for mastering.
Not perceiving form internally, someone sees forms externally that are blue [and now no longer dull or small], with blue color, blue hue, and blue tint. They’re like a flax flower that’s blue, with blue color, blue hue, and blue tint. Or a cloth from Bāraṇasī that’s smoothed on both sides, blue, with blue color, blue hue, and blue tint. In the same way, not perceiving form internally, someone sees forms externally, blue, with blue color, blue hue, and blue tint. Mastering them, they perceive: ‘I know and see.’ This is the fifth basis for mastering.
Not perceiving form internally, someone sees forms externally that are yellow, with yellow color, yellow hue, and yellow tint. They’re like a champak flower that’s yellow, with yellow color, yellow hue, and yellow tint. Or like a cloth from Bāraṇasī that’s smoothed on both sides, yellow, […] This is the sixth basis for mastering.
Not perceiving form internally, someone sees forms externally that are red, with red color, red hue, and red tint. They’re like a scarlet mallow flower that’s red, with red color, red hue, and red tint. Or like a cloth from Bāraṇasī that’s smoothed on both sides, red, […]’ This is the seventh basis for mastering.
Not perceiving form internally, someone sees forms externally that are white, with white color, white hue, and white tint. They’re like the morning star that’s white, with white color, white hue, and white tint. Or like a cloth from Bāraṇasī that’s smoothed on both sides, white, […] These are the eight dimensions of mastering.
For clarity, the colored cloths and flowers and such are not literally the things you see. These are just metaphors, or ‘imagine it’s like’ (seyyathāpi). They just try to describe the purity of the color of the nimitta. (On the order of colors see Wynne - Origins of Meditation p31: they were considered to be gradually more pure.) These limited and unlimited forms are also described in MN128, where they precede the jhānas, as I said.
So if this is all pre-jhana, how about form in the jhanas?
Along with the four primary elements, some suttas include in “form” also a fifth, namely space. In the jhanas “form” refers to a perception of this space (which in Indian thought is intrinsically connected to the four elements), not to colors or lights. Space is the most subtle of all forms, and in the formless attainments all form is abandoned, including the most subtle, hence the prior form in jhanas is that of space.
This is also why the first formless attainment is that of “unbounded space”. Here, as I conceptually understand this very deep state, the “concept” of space loses its meaning, so it’s also a perception of no-space. We could say it borders the form and formless, although it’s closer to the latter. In Mindfulness, Bliss, and Beyond, Ajahn Brahm describes it as: “In this absolute one-pointedness, space is perceived as both infinite and empty, a sort of no-space. […] The first immaterial attainment, then, is the mind-base of unlimited space, perceived as both infinite and empty, immeasurable and undefined.” I am even tempted to translate anantākāsa as “undefined space” instead of “unbounded space”, which isn’t even that crazy linguistically speaking.
Sue Hamilton’s Identity & Experience, which contains the most in-depth canonical study of rūpa I’m aware of, also makes a distinction between rūpa as the body and more subtle forms of rūpa such as lights and space. She also concludes the “form” of the jhanas refers to space. I’m not in full agreement with all she says, but I mention this to indicate that these kind of ideas about rūpa are not limited to a specific group of jhāna practitioners or the commentaries. You can come to this conclusion through a scholarly route also.
In sum, this is how the first stages of meditation happen: you focus on the nimitta, which starts off limited in size and dull. It becomes brighter and its edges start to disappear, becoming unlimited and measureless. During that process the body starts to fade away from awareness as well. Once you enter the jhana by letting go of the final subtle hindrances (which is a sense of self), you in a sense “enter” the nimitta. You leave the coarse perception of lights and shapes behind, only focusing on the most beautiful aspect of it, which is the pitisukha (which, rather than two different things, at that point is experienced as one thing with two aspects). There is still a subtle grasping of the mind at this pitisukha, the vitakkavicāra, which gets abandoned in the second jhana.
Then what I envision happens, is that the pitisukha, being at that point the coarsest aspect of the experience, gets abandoned as well in the third and fourth jhana. However, the “container” of that pitisukha, which is the space in the mind in which it occurred, still remains. In the formless attainments this space starts to fall apart too, hence the name ‘formless’. In the final formless attainments consciousness also starts to lose its meaning and fade away, into a perception of nothingness, then the state of neither-perception-nor-non-perception (i.e. so little perception you don’t even really know you’re aware or not), and finally then the cessation of all perception/awareness.
Since you ask, this absence of lights in jhanas is also what Ajahn Brahm teaches, though even if he didn’t, this would still be my view on the matter. I actually only found out his take on it after I already came to this conclusion myself.
I’m also not just echoing the commentaries, for these do seem to describe colors inside of jhanas. However, as far as I know they are rather terse on the matter, nor are they always 100% consistent. Their terminology is also somewhat alien to me (“such as jhana-eye”, what is this, jhana proper or just meditation in a more general sense?), so I won’t rule out completely that they disagree with me. I may just misunderstand what they’re saying. Or it may just be a disagreement on terminology but not on the experience.
Finally, to clarify again, since in the past I’ve gotten some not-so-kind private messages about these matters, all that is just my interpretation of the texts in light of my own practice. Anybody else’s meditation practice is not my concern here.