There are stereotypes about black men that make them seem pretty attractive.
AURURA: It's not the same for black women, though. There's a stereotype in Japan that black women aren't... well, the social idea of how a "woman" should act. I'm a black West Indian woman myself, and Japanese men and women have always reacted differently to me. Many consider black women - West Indian women in particular - to be rebellious of social ideas of how a woman should act. Of course, that can't seem very attractive.
On top of that, I think that Japan has adopted many ideas of Western racist ideals of beauty. The USA was supposedly against the Aryan ideals, but guess who will generally be considered most beautiful? That white man or woman with blonde hair and blue eyes. Japan has adapted that ideal. Of course, a black man or woman isn't included in that.
Who knows, though? I think societies around the world are starting to become more aware of subtle racism and sexism, so maybe once people are aware they'll change their ways.