im not sure about the nightmares
because although we can have nightmares, thats only cos we have seen things and have a rough idea of what a monster could look like

and iv just been thinkin more about the shapes that they can see and thought...
how can they dream them, if they have no colours, otherwise there will be no background to see the shapes on
if they only have one colour, lets say black for instance, it will be a black shape on a black background, which you wont see
i guess the only way to find out and answer this question is to ask a blind man