Why are some dogs scared of darker people?

Sometimes pets, especially dogs, can be picky. I had a dog and she greatly disliked men. I walked her somewhat often and whenever a man would pass us jogging or whatever she would cower and snarl. Same when I took her to the vet, and it wasn't pretty. She did, however, have a past of abuse with her old owner, which was a man. Your dog may have not been around dark-skinned people that often and think that it is unnatural to have skin that dark. He might think they look odd, but he should get over it soon, though. If you continue to take him on walks and he is around them constantly, he will most likely get used to them.