They don't actually use it in that way.
By the curb, they do. I knew someone blind and watched him tap the curb and street..but then do the sweeping motion you describe. I guess it would only matter if the manhole was right at the curb. Then again, I also saw him misjudge the depth of a hole and not step down far enough.
In general, blind people tend to step more carefully and notice when they come up to a hole. Therefore, it is still reasonable to assume that a blind person would be less likely to fall into a manhole than a distracted teenager. Though, it doesn't change the fact that open manholes are dangerous.
I still think the teenager deserved falling in the hole for being stupid. Though she could have been hurt, it would still have been extremely funny to see.