I don’t think this is 100% cannon but a lot of blind people are extremely sensitive to light. She may keep them closed for that reason. But also perhaps the author simply likes the mysteriousness of us readers never seeing her eyes.
I looked it up and apparently some blind people do actually keep their eyes closed and it can be for a variety of reasons such as:
-sensitivity to light
-lack of muscle strength
So it could be for one of these reasons she keeps them closed.
She did mention in one of the chapters after she woke up from a dream that she felt she had left the lights on (because of the warmth in the dream, I think?), and he said something like "so you could tell?" Don't know if that translates to light sensitivity though.
I understand that she’s blind but why does she keep her eyes shut?