I have been informed by no less than 3 professors in English that contacting professors before you're admitted is a big no-no and a waste of time, as it's not like you need to find an adviser to let you in their lab/etc like in the sciences. The impression that I got is that it's seen as ass kissing and not really much more than that.
I'm curious about this as well. My primary interest is the Caribbean, and while yes, that area varies a lot in terms of ethnicity, I do wonder if it "matters" that I am a white American female.
That said, the only prof at my MA institution that teaches Afro-American lit is a white male, and the only prof that teachings African lit (as part of a World Lit seminar) is a white female. I go to a medium-ish state school with no ph.d program.