Reverse mudsharking...
seriously though, they are the first of this kind that i have seen. You see the occasional slut that goes fishing for blacks in the clubs in Roppongi, but it's always only "for the story".
These girls are 20-25, they fuck around, then marry a Japanese man who will work every day from 7am to 11pm and that's that. They just want to have a little fun and have a story to tell that they once fucked a gaijin.
These girls are also most of the times considered ugly by Japanese standards, although western guys seem to like them.
I have never seen a Japanese man with a black girl in Japan in my life.