In the end it comes down to, (IMO) no matter how many black friends you have: If you see a black stranger walking, doing nothing in particular and your automatic first thought is, "that man is here, he is black, he must be up to no good because no blacks should be allowed to walk these streets without my express permission, I must call the cops to get rid of him although there is no evidence that he's committing no crimes that I can tell, except that he's black and he's in my neighborhood"... then you're a racist.
Friends are people you are familiar with and you may have overcome your prejudices with them, being confronted with the evidence that they're quite nice, but if it's a stranger you don't have anything to work with except your subconscious attitudes.
There are black people living in the residences, ¨they have children, they have relatives, they have black friends coming over. Some of the white people have black friends and relatives visiting them. GZ had no valid reason to think every black person is a burglar until proven otherwise but he did.
