According to some of the more benighted and ignorant creationists, the human genome can only be ‘degraded’ after the Fall of mankind. White skin is caused by mutations that render nonfunctional some of the genes that would make the skin black.
Thus according to that way of “thinking” blacks are not only the first humans, but also more in the ‘perfect’ original state of mankind - and whites have degraded from that more perfect state of humanity.
Also, I've read that black babies come out white, and turn black later.
Since dark pigment is a dominate phenotype, your theory doesn’t make much sense. The only way that this could happen is in a community in which female would have to produce a generation in which every offspring had a recessive genotype mutation. Not only that, this generation would have to be completely isolated from sexual intercourse from all other people, including the previous generation. How likely is that?
Perhaps Adam was white and Eve was black and their descendants ran the gamut of varieties of hue.
I though a mad scientist named Yakub accidently created us whites.