This is a good question... the first time i thought about this was actually when Oprah had a kid on her show that was blind from teh age of 6 or something (due to an accident), and she asked him what he dreams of... and he says he dreamt that he could see. She started to lose control there and started crying. Anyways, i was curious as to what somebody who has NEVER had a functional visual system would dream about... but i would imagine it would be things that they could experience, such as sounds that stir their emotion, smells that reminds them of something, tactile sensations that they liked, etc.
Sight is just another sensation, no different than hearing, tactile, etc. I may dream of a girl because i've been condition to like girls (i've come to link good looking girls with being something i want), so i visualize in my dreams a girl that stirred those feelings... a blind person would probably replace visualizations with tactile stimulation, perhaps they were once touched a certain way by a girl that turned them on, so in their dreams, they probably dream about reliving or wishing for that kind of sensation again. Sometimes when you close your eyes, you can dream about a certain music you've heard, and i'm sure blind people do the same thing as well... except just more often.