If someone has been blind since birth, they will only have auditory dreams. If they became blind, what they might see in their dreams would be built around their memories.

If you went blind as a child, you might remember some things, colors, abstract shapes, and dream those. If you went blind as an adult you would probably continue to have visual dreams for some time (hell of a dreamsign that would be). Either way, given enough time though, dreams would probably become almost completely auditory even for the adult.