African ancestry begins with your African ancestor. It seems as if you have a problem with the concept of race. You keep talking about percentages, mixes, and all of that but being mixed and black has never been mutually exclusive. A person with one white and one black parent has always been considered black. Yes, Halle Berry's daughter is "black" because she has African ancestry. You're trying use your personal opinions/rules concerning race and apply it to society. It doesn't work that way. You don't get to make up your own definition for what "black" means.
I don't agree with the idea that black is only good when mixed with white or something else either, but I don't see what that has to do with this particular issue. White people are racist, what else is new? Of course their going to give ligher skinned blacks preferential treatment over darker skinned blacks, that's how the system of white supremacy works. This isn't new, they were doing this during slavery. Again, the way to overcome their racism is to unite together. You still haven't explained how excluding people like Brack Obama from being black helps other black people. Imagine if Obama denied being black and went with that mixed bullshyt? He'd look like an idiot to both blacks and whites. White people know good and damn well that his mother is white. Do you honestly think that all of the whites who call him ******, monkey, and every other anti-black slur under the sun aren't aware that he's technically bi-racial?
If you have non blacks in your family, then you're own logic then you're not black either. Going by your "mixed people aren't black" logic, blacks are probably about 3% of the US population instead of 13%.