There’s also “animitta samadhi” (see e.g. sn22.80 & thread) so presumably this is a sort of samadhi that has no object or features - I don’t know if that is the silent illumination or mu koan of the chan tradition or not “What is animitta samadhi?” is a long standing dhamma question I have.
There are these three unskillful thoughts. Sensual, malicious, and cruel thoughts. And where do these three unskillful thoughts cease without anything left over? In those who meditate with their mind firmly established in the four kinds of mindfulness meditation; or who develop signless immersion. This is quite enough motivation to develop signless immersion. When signless immersion is developed and cultivated it is very fruitful and beneficial.