MN21 Kakacūpama. The Simile of the Saw. Discussion from Dhamma Wheel.
http://dhammawheel.com/viewtopic.php?f=25&t=1016
A very important Sutta for in it we find the term “anidassano” which is connected to Anidassana, therefore instrumental to understanding the meaning of “Viññāṇaṃ Anidassanaṃ”.
The term “anidassano” in MN21 is used explaining empty space as an unsuitable medium for painting pictures.
It is this absence of a medium for [painting pictures] and not the absence of Empty Space that is the key to understanding this term properly.
For Viññāṇaṃ anidassanaṃ
I would translate it “Consciousness without a medium” 
As i see it, a medium for what it should be asked and the answer is a medium for arising of “existence/suffering” [sankhara (conditioned phenomena]
So it is The Dhamma without conditions for arising of any Sankhara.
Sakharas arise of&from Sankharas.
Because it has been proclaimed and i translate;
Sabbe saṅkhārā aniccā. - All formations are impermanent
Sabbe saṅkhārā dukkhā. - All formations are stress
‘Sabbe dhammā anattā’ - All Dhamma are non-self
Therefore All Sankharas are Dhamma but not All Dhammas are Sankhara. There is a Dhamma which is does not share the characteristics of Dhukka & Anicca with the other Dhammas.
Which Dhamma it should be asked, the answer is in the MN115;
There are, Ānanda, these two elements: the conditioned element and the unconditioned element. When he knows and sees these two elements, a bhikkhu can be called skilled in the elements.”
Therefore the only possible referent of this term is the Unconditioned Element [Nibbana] and nothing else.
i think as far as conveying a meaning it is a pretty good translation and actually has an explicit & rational semantic root in the Sutta