If you are black you are black, if you are white you are white, if you are wearing a red t-shirt you are wearing a red t-shirt. Anyone who gets offended by a simple adjective like that is so entrenched in the ranks of the PC brigade I don't want to be on good terms with them anyway
It's not offensive, it's a skin colour. It's easily recognizable and useful in situations like the OP said.
If saying black is offensive then so is saying white and no one has an issue with that.
We are all the same, skin colour is nothing but skin colour and shouldn't be made a big deal out of. If you need to identify someone quickly skin colour is a big help!
I only say "African American" if it's a professional situation, such as school or work. I don't think that most black people mind the word "black"; atleast none that I know of.
Before I resort to describing them by their skin, I would always find it convenient to describe them by the clothes they wear often and such. But I use, black.