I had a whole thing typed up about black people and dogs, then deleted it because it sounded so fucking weird; then I saw your post, so here goes. This is one of those things common with rescue dogs as well; it's not something anyone who works with those animals is going to talk about, but anyone who has worked in the field knows about it. Some of the sweetest dogs in the world lose their shit around black people. It is not always aggression; some act terrified. I always assumed it had something to do with a smell we couldn't pick up on, but not being able to read their faces makes sense.
Yeah, it’s not uncommon that some dogs, especially non neutered males, just seem to hate men, but even then it is all men and often not male children.
It certainly never seems to be the case that a dog develops an “irrational” hatred of white men or white people.
A couple of my dogs have been uncertain about south Asians (Indians/pakistanis), and I have mentioned this before but I had a deaf Dalmatian who really seemed to hate drug dealers/people on drugs/people likely carrying drugs.
Some of these people surprisingly also happened to be black

, but he never reacted badly to black people who were just minding their business.
I overheard a couple of west african and very black guys saying that he looked like a spotted hyena, and he definitely looked at them but didn’t care.
So maybe its just particular dogs, but in the case of the south Asians, they tend to collectively be wary of dogs due to pet dogs not commonly being a thing in their culture, so I think it was more the dog picking up on them being wary.