I'm not black but I have a couple of black friends and a few that are in my professional circle. We're not BFF's or anything of the sort, but we're casual enough to discuss dating and who's fucking who.
My professional black colleagues that aren't married already, have successful dating relationships with other professional J-ladies and other non-Japanese women. At least one friend has clearly communicated that he doesn't have much luck except in meeting women that are in the [black] gaijin hunter group.
IMHO, it's more difficult but certainly not impossible. Black guys have a much better reputation than those from SEA or those native to India. You will find women eventually that may be exclusive to one race of men and will shut the door in your face if you don't have the right 'look' or skin color.
Fair warning - depending your nationality, you'll face an uphill battle with some that associate you with Nigeria by color only. If you don't know already, we have a fair amount of problems from Nigerians and that problem doesn't seem to be going away soon.
I can only speak exclusively for the Tokyo/Yokohama area; don't have any friends or colleagues in the Kansai area as of right now.