I think blind people do indeed see in their dreams.
To answer your question, blind people see darkness because their eyes.can't perceive light. We can see darkness with our eyes wide open. Go into a room at night and block the windows and turn out the lights. That's how a blind person sees.
Blind people use sense of touch to work out what a persons face is like and then bring up a mental image of what its like.
Better yet, what does a person with no eyes see?
You would think they see black, right? But, no. We see black when we close our eyes because we're looking at the backs of our eyelids. My curiosity of this is driving me crazy! Even if they see blank, even blankness would be made up of colors, right? Which you need vision to see. WTF!
What does it even matter, as someone above said Google it, since who will know the answer here?
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Even Better...What do they dream about? Can they see in their dreams?