Why does some those armpits turn black and how do you cure it?

Why does some those armpits turn black and how do you cure it?
Pick up some white-out at your local paper and organization supply store.
Black armpits?! Never heard of that one, unless you be suffering from bubonic plague.
Seriously though, it could be a impulse to a deodorant spray or something. Just get it checked over and find out in the order of it from the doc.