Was wondering, was your grandmother's original reaction mostly because the dude wasn't Dominican, or was the possibility that he could've been a black American that contentious? Also, in your opinion, how do you think she would've reacted if he was an Afro-Latino from a different country? Or even an African?
As for the OP, this shouldn't surprise you. Exceptions aside, Asians especially tend to stick to their own.
The bigger question, IMO, is why do black Americans (and by extension black people in general) have that bad of a rap? And what needs to be done to tackle that problem.