In general, fewer people are bothered by "black American" than "African American." Many are okay with the latter, but that's not safe to assume. The point I was making is whose ancestors lived in Haiti for the last several centuries often won't identify with Africa.
The more significant point is that it has undertones of the idea that having a black heritage "taints" someone. Some people with black skin are 50%-75% European. Calling them African instead of European reinforces the idea that having any black ancestors overrides everything else, which bigots often think.
Even if that's not your intent, it's better to avoid the issue entirely and say they're black as an objective statement about their skin color rather than African a comment on their heritage.