The main thing is not to feel awkward about it. If you wonder whether a blind person needs help, ask. Then, if the person says no, let it go at that. So far, I have talked about help that has been courteously offered and probably should have been accepted. Blind people don't have a monopoly on rudeness or bad manners.
Sighted people are human, too.
I think of a time when I was standing on a street corner in Des Moines, minding my own business and waiting for a friend. A big husky fellow with the momentum of a freight train came along and scooped me up without ever even pausing. As it so happened, I didn't want to cross that street. I was going in another direction. But he didn't ask.