I'm so sorry to read this abt your kitty, didn't the Vet tell you what was causing this, wht exactly did the Vet say to you?
A kitty I had but has since passed on got a red eye that turned out to be high blood pressure, he ended up going blind in that one eye also. but what you are describing sounds much more serious....
He said that it's a virus came from another animal caused him fever then it damaged his health, also said he won't get his sight back, and the reason why he's so slim and has loss of appetite is he has a trauma because he's completely blind. I can't trust him cause the veterinarians in my area aren't so good I'm thinking to send him somewhere else.
Thank you for your time and for helping us, but it's no use :( you're right seeing other vets won't help. Thanks again I really appreciate it