I’m being a cheater and coping a previous answer on this thread (in its entirety, because I’m too lazy a bastard to cut out the irrelevant parts).
“Great question. I’m a bit busy, but I’ll share an experience with you all, might give you some insight.
“Lucid Dreaming (this is the elementary explanation – stay with me, oneironauts!) is the act of fully controlling your dreams. There are many ways to achieve this, and with practice, you can pretty much turn full control on and off.
“Well, one night I was experimenting with what I can do. Somewhat bored with flying around like Superman (yes, it’s what everyone does first – it’s quite difficult to reach the point where you can do this without losing control of your lucid state), I decided to try a bit of a test.
“We perceive the world in three primary colors – red, blue and green. Every color in this world is some combination of the three. I wanted to see if I could have my subconscious create a fourth primary color. Our brains are written to know these “natural” colors; what happens when you force it to imagine another color?
“Well, I did just that. I created the scenario where I was standing in a field alone (quite a common scene for me, I generally use this as my starting point when creating a world). I stared up at the sky, and saw it was a deep blue, with no moon, sun or stars. I shifted that to red, then to green. I started to mix colors and came up with yellow, browns, purples… any color combination I could think of. Eventually, I told myself, “Now, a fourth primary color.”
“And I did so.
“To this day, I still cannot describe the color, as the only way I can is by using other colors as examples (I sometimes try to describe it other items and still fail). As @YARNLADY has said, I know that my subconscious mind was creating the color based off of something, I just don’t know what. It’s a mystery to me, one I’ve been trying to solve.
“So, to connect this to your question, maybe a blind person “sees” things we cannot, as their minds are using reference points that we can’t understand. They’re other senses are heightened and trained to make up for their failed vision. Therefore, I can only assume they “see” in a very different way, and imagine something like “primary colors” (something that can only be described to them, something they cannot witness) in the way I “saw” that fourth primary color.
“A chair might look very different to them – they have felt it, know the outline, but cannot encompass it from afar. They know of their dog, yet cannot “visualize” it they way we do, and probably “see” it very differently in their mind’s eye.
“Difficult, I hope someone can give us a better answer.”