From this NY Times article:
It’s interesting that those middle class families that are complaining about unfair treatment don’t realize that they’re being judged according to often accurate stereotypes, such as the one black nanny being worried about black neighborhoods being more dangerous. So while it may not be true of them, it is natural for humans to make associations that effect self-defense. This doesn’t make it pleasant, but they shouldn’t be surprised.
I’ve seen this black vs. black prejudice in other places as well — namely in the restaurant and taxi businesses. It’s really just simple human nature: both of these industries rely on tips, and if you ask 100 black waiters or 100 black taxi drivers if they prefer black or non-black patrons they’re likely to mostly prefer non-blacks. Why? Those I’ve asked say that blacks tend not to tip much, if at all.
Don’t be surprised about human behavior — look for the incentives.