A wide variety of things, because not all blind people have the same level of vision loss. Even people with the same effective vision loss may have totally different subjective experiences due to different types/areas of damage. However, for the sake of the question I'm going to assume you mean totally blind people. The answer is nothing. It's not black, it's just nothing. A common way of getting people to understand this is to say: "A totally blind person sees out of their eyes what a sighted person sees out of their elbow". If you try and focus on the visual information coming from your elbow, do you get darkness ? Of course not. It's simply nothing st all.

Yeah, probably not I can because I use to see a tiny bit. So yeah. Also depend how much one pays attention to forming sensations. Or if they are more dreamy.

I once heard someone describe being totally blind from birth as being the same thing that you see when you look out your elbow. What can you see when you look out your elbow?

Nothing.