Funny, most answers come from men. It's true the color contrast is attractive to everybody, including, I believe, for black men. But as someone explained here, it's the expectations about IR experiences which actually create some strong craving for well endowed black men. In a way, it's the woman who makes it an obsession for herself, as we unconsciously chase always better attributes than on the last man we had. Compared with other fetishes, yes, changing partners adds an extra dimension to any other sexual practices.
Perfect comment. One reason a white woman is interested in a black man, other than his race, is the pursuit of of those traits that surpass the other men she has been with. My wife has the exact interest in black men, she loves there skin color but she is interested in a black man that is well endowed that can give her a new experience in bed.