As you say, it's not all of them. But it's enough of them.
The problem with blacks being unable to catch cabs is how much black cabbies distrust other blacks. Years ago that has-been actor Danny Glover tried to make a stink about this, and the story died quickly when investigative reporters noticed that most of the cabbies passing blacks hailing cabs WERE BLACK THEMSELVES.
When we returned from living in Africa for a number of years, our kids were enrolled in a 90+% black public school. Our 8 year old son was the only white boy in his class. My wife asked him how school was going and did the other kids treat him?
"Do they treat you with respect?"
"No, ma'am. They don't respect me, they don't respect our (black) teacher, they don't respect each other. Someone needs to put them on a boat and ship them back to Africa to learn manners from the African kids!"
Our kids grew up in Africa playing with African (and 'European' kids). Our son was the first white child our tribe had ever seen. While in college, he was made an honorary member of the African Students Association and wore their sash at graduation. He definitely is not bigoted!