As a matter of fact, we once had a German shepherd who went berserk when he saw blacks. We got him at the pound, where we were told that his previous owner had kept him chained to the back stoop until he was evicted and had to get rid of his dog. Evidently the dog was mistreated by this previous owner, who was evidently black.
On the other hand, he also had a fit when he saw someone wearing a hat. So he wasn't just a racial bigot.
Needless to say, we had to keep a close eye on him. But he lived a long, productive life and loved our family.
We had a German Shepherd who went ballistic when she saw uniforms, ANY uniforms. The meter reader, the postman, the milk man, police, fire....you name it. If it was a uniform she became Cujo. The rest of the time she was totally passive. Never could figure it out.