I suppose they'd see everything as different shades of greyscale; I saw a video done by this blind Youtuber once where he answered the question "Do you see when you dream?" He answered no, and went on to provide an example of dreaming about a horse race: he would simply hear the galloping of the horses and not see anything, just like you would expect but without the visual sensory input.
Likewise, what a blind colour-synaesthete sees would still seem perfectly normal, to them at least. That is assuming they were colourblind from birth, though. If they went colourblind after birth, after seeing colour at least once, then it'd be a much sadder story in their terms. "Oh woe is me, colour and all that, etc.," whereas the term "ignorance is bliss" rings true for colour-synaesthetes colourblind from birth.
Still, if I read a black text then it's still coloured for me. I can "see" the colour, but I can also "see" the fact that the text is black. I don't know, imagine 3D glasses where you can see blue and red at once but they don't mix to make purple. It's like that, except the colour I see is a lot more vivid than the colour the writing is physically written in.
