People who were born blind never saw anything, hence they wouldn't see dreams. They do not retain visual imagery because it was never acquired in the first place. Dreams are not only visual in nature, its an amalgamation of one's senses. Blind people have heightened senses of hearing, touch, smell, and taste, because of their lack of sight. Their dreams are based around the senses they use most often, just as people with sight have dreams based on their sight (because we use that sense most often).
Our dreams reflect our waking life experiences - conscious and unconscious. If someone has been totally blind since birth, they may only have auditory dreams. People whose vision has been blurred from birth dream of blurred images, people who have been deaf from birth do not hear in their dreams, and the colourblind are equally colourblind in their dreams.