Dogs are situational, and they're suspicious of anything or anybody that's "different." If a dog has never seen a black person (or a white person for that matter) the "difference" makes him suspicious. Depending on his personality, he may be fearful, or aggressive, or both.
I've got a Chocolate Lab who loves EVERYBODY, red and yellow, black and white, they are precious in her sight (who knows, they might have something good to eat.) Of course, she's been around all kinds of people since she came home with us at 12 weeks.
She doesn't like the UPS man who delivers to our house, though. I think she thinks he's carrying packages away rather than delivering them. She gives a good impression of Crazy Killer Dog while bouncing up and down inside the kitchen door so that she can see out the window.
A week or so of running around on the ranch and she calmed down somewhat but still stayed 25 feet away from me. Women were no problem but even with them she didn't want to be petted or hugged.
Then she had a problem with foxtails caught between her toes, feet became infected. When it got to the point where she could hardly walk I had a lady friend hold her down while I surgically removed the foxtails from all four feet. Applied some medication and bandaged them. Then she became my dog!
I could pet her, wash her, hand feed her, you name it. Figure that one out. Hate me then love me all in 30 minutes!
Move on five years, as far as I know she had never seen a black person. While waiting to unload in Portland I took her for a walk using a long leather leash. Six warehouse workers were walking towards us on the sidewalk. All were black.
She stopped, looked, when I got beside her, she forcibly pushed on my leg moving me to the building wall. The hair on her back and neck stood up, I grabbed as much leash as I could thinking she might try to attack these men. Instead she just stopped, waiting for them to attack her!
The men walked out into the street around a couple of cars before getting back on the sidewalk. All the time I'm reassuring her these men weren't from Mars and didn't mean her harm, apologizing to the men at the same time.
'Til the day she died she did not like strangers of any size or color. I was the only one she would retrieve a ball for. I was the only one who could touch her stomach or check her teeth.
Whatever happen in her life as a puppy had to be terrible.