Nonsense, says Dr. Nicholas Dodman, a professor at Tufts University School of Veterinary Medicine and author of several books on animal behavior, most recently If Only They Could Speak. “Any behavioralist knows that dogs don’t like subsets of people,” he says, and though the most common subsets are broad—strange men or little children—”sometimes it can be quite specific. It could be tall men, or men with beards. It might be men who are wearing big shoes, might be as subtle as men who smoke cigarettes—which can be hard to pick up on—but it can also be black guys.”

https://slate.com/news-and-politics/2003/02/can-dogs-be-racist.html