“Hello is it okay if I sit by you?” Or any other normal introduction on a train you would make to anyone should do the trick. Just tell her to treat him like any other person.

Anything that in your best judgment would work with a cute sighted guy will also work with a cute blind guy ;-).

Of course there are certain differences that come up once you begin a conversation but most of them can be dealt with easily if you've got a decent amount of general human sensitivity. Here's an example of what I mean: let's say I'm asking a stranger for directions. Some people are kinda dumb and they reply something like "it's over there" when my white cane should make it obvious to them that "over there" doesn't tell me anything. Other people are great about it and they describe the way in an accurate way (with some landmarks thrown in) so that I can find what I'm looking for. Overall, women are generally better at this than men in my experience. But like I said, there's no point in giving you specific advice because there are so many potential situations where something like this could come up. You don't need to be afraid though. If you are an empathetic and curious person, it almost certainly won't be a problem.

P.s. Just one more thing I thought of: blind and visually impaired people often give off a vibe of insecurity. Many women find this unattractive in my experience. Don't let yourself be fooled by this though. Most blind folks are just as confident as sighted people, we just seem to be insecure because we're almost a tad worried that we might bump into someone or hit our head somewhere (at least it's like that for me).

It really it wouldn't be different. If she goes into this treating him like any other person, even if he's not interested, he'll probably still appreciate that, since we so often get treated as weird/outcast/damaged/eccentric/odd, etc.