Can you see something obvious going on with the eyes? Cloudy? Spreckly? Scales possibly growing? Did they receive possible damage by fighting? Or netting?
I think I'm going through possible "eye cloud" because my female HMPK seems to be losing her eyesite too. I don't see anything too obvious on her eyes, but I can tell that something isn't right. Anyway, I've learned that a bacterial infection can attack their outer eye membrane, causing them to start losing their eyesite. This can be bacterial or caused by damage. Darkmoon17 posted some responses to others with this issue and advised two things:
1) If it's eye cloud or damage, then start with a 10-day treatment of 1tsp/AQ salt per gallon for 10 days (100% water changes depending on your size tank). If it doesn't clear up, then bacterial meds would be next.
2) If it's the beginnings of pop eye, then you need to use Epsom Salt and I didn't read through that medical treatment. My girl isn't showing signs of pop eye (yet).
Usually true blindness is caused by severe damage, old age or ammonia burning. However, the most common eye issues start as a water parameter problem (bacterial) or damage. To answer your specific question, I have no idea if blindness can be reversed.
I hope I've helped in some way. Just throwing out what I've recently learned about eye sight problems!