I will occasionally have that darkness too, at least whenever I'm trying to DILD. When I'm in it, I'll know I'm trying to fall asleep. I won't be fully conscious, but enough to know where I am and not be able to fall into a dream again. To me it seems....hmm...how to describe it....fuzzy. That's all I can really think of.

All I can recommend is open your eyes. It will refresh your consciousness and not leave you fuzzy. Also, it could turn out that the blackness was really a dream. That happened to me once. I decided to open my eyes, then RCed and realized I was dreaming.