That's hard to describe. Not so much black, as nothing. When your whole visual cortex shuts off, the concept of color goes away. Basically, the mind falls apart untill there's almost nothing left, and then oblivion.
I'll be honest, dying wasn't desirable until it was inevitable. When I knew that everything was done, and I wasn't going to come back, it was a great relief. Everything shut off, and there was nothing.
Then it call came back, and it was hell. Of course, that was death by suffocation from heart failure, not exsanguination or drug abuse.