"It's a side-effect of the vibranium" is enough in comics-speak. How did the races originally separate? Millennia of selective breeding and isolation, sure. But also environmental factors. Vibranium gives off energy. That is canon. It reflects energy. Captain America's shield can deflect the black-hole-heavy Thor's hammer because of this trait.

It's generally agreed upon white skin is helpful in absorbing the sun's rays (or unhelpful, if you dislike sunburns), hence why it's more common in areas of less sunlight. Vibranium can cure any disease with its energy castoff, it seems. It's magic-ium by another name. Wouldn't the rulers WANT to be absorbing as much cast-off radiation as possible? They'd selectively breed, accidentally, into paler skin tones.

He's black because stereotyped opinions on geography and race causations, but he could very well be white because Vibranium if the creators wanted him to be.