They don't see. Anything.
Even when we close our eyes, we still SEE, but we see the black and red of the back of our eyelids. They don't see. They don't know what it's like to see. They don't see colors, they don't see blackness... they just don't see. When you are blind from birth your visual cortex is converted to a different sense, mainly touch and sound. This is why blind people can read braille so easily while we just feel a bunch of bumps. They essentilly lose the part of their brain responsible for sight; it's taken over by another sense, giving them much better perception in that area.
It's really hard to comprehend not seeing anything, not even blackness, but that's what it's like. Definitely bows your mind thinking about it.