"Sight" is not something which exist in itself, it's most like a name we created to mention something which doesn't exist but that almost every human being possess instinctively. In practice, what we call "sight" is the result of what our brain manage to analyse from all the stimulis coming out from our eyes. We "normal" humans take this for granted, and this is why we "see" in our dreams. Mainly because that's most of what we perceive in a day, and the dreams are only the reflect of what we know and a construct of what have or may happen in our life blended with emotions (such as fear, love, joy, etc).
Blind people can just "see" as well as anybody, it's only that their eyes don't work either at all or not good enough and that they don't have a normal "sight" like us. However, it been proved numerous times in the past that when a human lose one of their senses, the other senses tune up to compensate.
Like Morrin said, they probably dream of things just like us but in a different way. We could also hypothyse that even if someone is blind and never seen something or someone before, they can still create the image in their head just like another human being and still "see" something or someone in their dreams. Like I said above, the other senses of the blind people tune up a lot to compensate for the loss of the sight, and it's common knowledge that blind people can construct an image of something by "examinating" it with their hands. I guess it would be the exact same process than if you'd imagine a rotating cube in your mind (common mind projection excercice), but at a much, much bigger scale.
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