What do you think blind people mean when they say they see 'nothing', not even black?

I saw this near death experience documentary thing, and this blind woman had one (which she could see in for the first time and everything).
But she said that she sees nothing at all normally, not even just a black space, or in dreams either…
I was having a hard time imagining what it would be to see nothing at all (I dont really wanna find out for myself either), because i always imagined like the black screen or the inside of your eyes or something…
do you have any ideas or anything?