You're right over the very short term, but when you bring up the child of a black man and white woman as a counter example, you clearly can't see the forest for the trees. You understand Mendelian genetics, but what you're missing is evolutionary pressure. The very fact that black man is having children with a white woman, and not with a black woman (who is likely single instead) means the evolutionary pressure is away from darker skin.
And about Europe, you totally mis-understood me. I didn't mean that Europe has become lighter skinned over the last two years or something, I meant the fact that "white people" come from Europe. White people evolved not because god created them or by random chance, but because 10,000 years ago it was all covered in ice so there wasn't much food around, men could only afford in Europe to have one wife. So their (Men's) preference for lighter skin lead to the evolution of white people in the first place.
Anyway I didn't mean to stir up a hornets nest, this is just something I've been thinking about and as a black person it bothers me... I guess it shouldn't since black white we're all people and if black people over generations become lighter so be it.
PS. If you want scientific sources there's plenty online:
http://www.sciencemag.org/cgi/content/summary/316/5823/364a
EDIT: Also you're confusing concepts, the population does not become darker if for example all black men only had children with white women. It actually becomes lighter, there's more people that society would consider to be "black" but in reality the genes that code for dark skin are rarer.
Also I'm here because I do date white women, and the white women/black men phenomenon is something I've been thinking about so I thought this would be a good place to discuss it.