How do you "explain" to someone who was born blind that he is "blind"?
"Do you see blackness all the time?"
"No, I don't know what blackness is; I see the same as you see out of the bottom of your feet".
A blind person who has a sense of the size of a room, have they any idea that windows are transparent and have a view over a vast area of cityscape/fields/oceans/length of road/etc?
Standing on a hilltop or in a viewing tower, you can see people in the landscape, but too small to see their faces or identify them - still identifiable as people by shape and gait.
I often automatically turn to Google images for an idea of what something I'm unfamiliar with "is", having to revert to a dictionary and textual description would feel like a step backwards, and that's a very recent (last 5 years) development.
You could also ask if blind people are able to perceive vision in dreams. To which the answer, I believe, is that they simply cannot, due to the fact that their brains have no notion of a visual image.
Those who've never seen, or who were blinded very early, have auditory dreams. (They dream in sounds.) Those who were blinded after the age of around seven -- when the ability to form mental images necessary for dreaming develops -- are able to dream in pictures.
One can argue that 'seeing' goes beyond the perception of lights and colors. It's also about shape, so I guess a blind person can perfectly 'visualize' a rectangle, a cube, etc. And probably do geometry, or even 3D-object rotations.
Can anyone confirm this?