When a blue eyed white person and a brown eyed white person have childre, they can produce children wither either brown eyes or blue eyes. Even if all their children are brown, and all their children's children are brown, the blue eyes can still resurface and be expressed in their children because the gene for blue eyes is still passed down.
Well, why isn't that true for white people and dark skin? All humans are supposed to have originated from apes, and we all had dark skin. Somewhere along the line, white people deviated from that line and had a mutation that made them have white skin.
The question is, why aren't the genes for dark skin ever resurfaced in white people, the way that the blue eyed gene resurfaces in a family of brown eyed people? It's unheard of for two white people to produce "a black child."