A study by a Danish group suggests that people blind from birth do not see anything when they dream. They do report other sensory experiences (taste, sound, etc.) though, and with greater prevalence than sighted people.

In addition, the longer a person was blind, the less they "saw" in their dreams. Alternatively, other work suggests that blind people may still be able to have visual dreams even if blind from birth. Since those people do not know what "seeing" is like, it is not currently possible to say that they see anything during dreams though.

[I also found a response, by a blind person, to this same question on another site. The answer is copied below:
" Yes, blind people do dream. What they see in their dreams depends on how much they could ever see. If someone has been totally blind since birth, they only have auditory dreams. If someone such as I, has had a measure of sight, then that person dreams with that measure of sight. I still dream as though I can see, colors included. For people I've met since, their faces are just blurs or how I imagine they look. To me, someone like my mother looks forever 30." ]

"People who were born blind have no understanding of how to see in their waking lives, so they can't see in their dreams. But most blind people lose their sight later in life and can dream visually. Danish research in 2014 found that as time passes, a blind person is less likely to dream in pictures. Sep 18, 2014"
Source: BBC News