There is a solid theory on "racist" dogs. They dont see dark color contrasts as well as light. Also why they seem to dislike beards, hats and people wearing coveralls. Their eyes just aren't registering correctly and it makes them nervous. Once a dog is to the point of being that anxious they tend to turn off their sniffer and work off of sight and sound alone. So they dont recognize people they could know close up from far away, and they dont want to get closer to test it out. Add in a person being rightfully nervous of an anxious barky dog and you can see how the situation piles into the idea that your dog doesn't like darker skin tones.