Yup, thats pretty much the reason why it exists in Japan.
Its suppose to be taboo. Sort of like something you shouldnt.
Like "oh, look shes having sex with "that" " sort of scenario.
Back in the day there was sort of a thing to also have Caucasian males. But as I said above, it doesnt really give off the same vibe as they are trying to portray with "blacks."
Its gross.
But there has been 3 titles with a Caucasian man in the GG series. 4 if you count that one blow job scene only.
I could link you to these if you would like.