That's a tough one indeed.
Its been a long time and they're still not settled, eh.
The problem with blind kitties, bless them, is that they dont/cannot monitor their body language and behaviour around other cats. Which they really rely on socially.
Sometimes its very difficult for a sighted cat to understand the intentions of a blind one, because much of the mutual understanding is missing. This uncertainty can certainly make it difficult to accept the other cat.
Have you been working with positive scent association?
One good way might be (if you have not already) to buy a liver paste, and start from scratch by rewardig her when she sniffs something that smells like him. To associate his scent with something marvelous.
And the same with the boy.
I'm not sure allowing him to flee/exist upstairs will benefit him, because he has no choice but to avoid the sound, he has nothing else to rely on after all. Cats that hiss, do this to warn other cats. But its not necessarily aggressive. Because he sees no body language, he will assume it is aggressive, because he's safer that way.
If she hisses, but is not aggressive, I do think you should give him that extra gentle force/encouragement by keeping him in the same room as her. The hissing may not stop otherwise, because she feels triumphant when he runs away.
By keeping him in the same vicinity as her, you are telling her that he is here to stay and that she must accept it. If she can accept him eventually, the hissing will stop, and he will be much happier.
