Nah. All girls vary in how they taste. It depends a lot on things like diet and hygiene.
If you've made this observation, I'd say it's most likely:
1. If you find yourself more attracted to black girls, you're probably going to be more turned on when you're with one and thus, find going down on her more enticing.
2. It may have been true of the specific girls that you went down on, but I doubt your "sample" is big enough to be representative of all black and white girls.
Or, I suppose it's possible that there might be a cultural difference in diet between the black girls and the white girls you've been with.