Have you trimmed her crest just to make sure that it isn't the fact that her eyes are covered with feathers. I know that can happen with polish and it does cause them similar problems to being blind. If it's not that then being blind is definitely something that happens and I've read that blind chickens can live long happy lives they just need some help at times and extra attention. Also I wouldn't let her free range if that's something you do because she will basically be a fast food snack for predators.