But if they were all blind, why would they have eyes in the first place? That doesn't make sense from an evolutionary standpoint.
Well, perhaps the lower species they evolved from had eyes but with climatic and cultural changes, the need for vision became less important to survival, so eyes became less functional but not removed altogether, a more obvious analog to the appendix. After all, we have moles on Earth who evolved from fully sighted creatures but whose own eyes have almost withered away - still present but mostly useless and unnecessary because of their environment.
Or maybe they have eyes because their genetic heritage from the aliens in "The Chase" demanded they have eyes, even if they didn't really need them.
Or what DerangedNasat
All of which is moot anyway, because the whole idea of them being blind appears to have been dropped.
It may not be moot.
A few things:
1) From what I can see, 23rd century Efrosians seem more likely to have been depicted with vision problems than their 24th century counterparts. Perhaps Federation science came up with a simple fix for chronic vision problems in Efrosians in the interim period. (Might be something as simple as a vaccination against a common virus that targets Efrosian eyes.) This sort of thing isn't unprecedented; 100 years ago in the US, polio was a major concern. Now, no one worries about it here. Maybe it's like that (which would explain why no one ever talks about it; why would they?).
2) Didn't the Efrosian Ice Age occur even more recently than ours did? If it did, and if Efrosian blindness was related to it, evolution wouldn't have had time to eliminate the eye, particularly if Efrosian civilization was developed enough to severely limit the power of natural selection (which it sounds like it had). Like lvsxy808 said, losing vision just might not have been that big a deal for them, and so no one bothered to correct it while the civilization was limited to Efros Delta.
3) Efrosian eyes could also simply be naturally much poorer than human eyes (and always have been), and as such, they are "blind" by our standards. By canine standards, humans are anosmic (unable to smell), but few people worry at all about improving our sense of smell. So the Efrosians might have felt that their eyes weren't anything to worry about, and it was only after they as a species had spread enough through the Federation to be at a substantial disadvantage that some form of vision enhancement (biological, prosthetic, genetic, I dunno), which Xin would appear to have received, became important to function.
I'd be curious if Starfleet Academy had some sort of visual acuity requirement in the 24th century (the way it has other physical fitness requirements), and/or some sort of auditory acuity requirement. How would that work with the ban on genetic engineering? Could you use prosthetics to increase already normal-for-your-species abilities?