I've seen the same thing wth dogs and black meter readers at my work. And I knew the dogs in question and their behavior was quite unlike what I had seen before. I think it is caused by the dark skin and that the dog is somewhat confused by it. I see the same exact reaction with people wearing brimmed ballcaps and sunglasses. Take them off and the dogs attitude changes. Maybe they don't reconize the person as a person at first, and since they are not familiar with the odor of the person they are going on sight instead. I doubt it's racist, more like confusing to the dog who then becomes defensive. However, I have also seen people who's BO sets off dogs for some reason, and the person [who usually is a bit frightened of dogs with good cause] tends to start sweating from stress when they meet up with poochie, escalating the problem. Heck, for all I know, there is a dog gene that at fault.
As for me, I am a dog magnet, the vast majority tend to act like I've been their master for years, something that I can't explain and won't bother trying because I don't know, except for the BO thing again.