Merriam-Webster defines fetish as:

an object or bodily part whose real or fantasied presence is psychologically necessary for sexual gratification and that is an object of fixation to the extent that it may interfere with complete sexual expression

Since an object must be involved for it to be a fetish, strictly speaking it's not a fetish you have - unless it's a particular body part that turns on and I won't speculate on what may be Ass fucking is a sexual activity so I guess that would be a sexual deviation.

But what it's called is irrelevant, we all know what you mean and it seems just a simple case of black guys not rocking your boat. Doesn't sound anything strange or particularly unusual to me, Most people have an inexplicable aversion or attraction to something. One that springs to mind for me would be an aversion to the accent from certain cities in England and attraction to other accents.

Anyway, nothing to be worried about and personally I think it's a good "fetish" to have. Of course me being white doesn't influence my opinion in the slightest