What is the best way to handle someone who's unintentionally racist?

The problem isn't with your friend...it's with an overly sensitive society. Saying someone is black is not racist...it's just fact. Buying into stereotypes about certain races is also something we all do...some people just don't talk about, while others do. If YOU ran into a big black guy it would scare you too...the only difference is you don't voice it while your friend does. The solution is for society to just lighten up and quit interpreting every little comment as racist and bigoted.