Um... I guess you are unaware but it is certainly possible that a black couple has a different skin coloured child (the chances are low but not impossible). Thats so because if anyone in their heredity (mother's or father's) was white skinned, the white skin gene would be present in all off springs along with the black gene. The skin colour is decided by which gene is dominant. In their parents, the black gene is dominant, so they are black. They have the white gene as a recessive gene. This white gene can be passed onto their offspring. So the white gene transferred to their daughter, along with the black gene but the white gene was dominant in her thats why she was white and not black.
For those who dont wanna read the whole thing, if any person in a man/woman's heredity was white, while rest were black, then any offspring in the future can be white too just because of that one white person.