If I think about it from an experiential aspect it's not like I just blinked my eyes. There's a distinct "knowing" I was out for some time. Advaitans will say Consciousness was still present and it was just the mind that was offline, and since the mind presents experience to Consciousness it's an awareness of absence of experience. So Consciousness is conscious of a lack of experience since there was no mind to construct and present experience to Consciousness.

Deep dreamless sleep and Samadhi are the same thing. There are times during a particularly fruitful sitting when the timer goes off it's almost a blank, though I'm keenly aware time has gone by, just not much experience to recall. Experience of absence.

So for me it's not even an abstract concept but something that's actually experienced.