From our experience it is a dominance/submissive situation where anal sex is the final and total submission to a lover. Amie and I had been married for several years before I shared her with black men. Up until then she had never allowed me to take her ass, claiming it was too uncomfortable for her.
Needless to say I was shocked when I watched her have sex with a black man the first time. I couldn't believe it when she willingly and eagerly let him invade her backside with his big bare black cock. She told me afterwards that when she is with black lovers she is willing to do allow them to do anything they want to her with nothing off limits. She feels so submissive to them and the more dominant they are the better. While I cannot speak for the black men she has been with, I imagine they sense her submissiveness and anal sex is the last boundary to be crossed to total domination.