....heck, some black people argue that Barack Obama isn't "black." :lol:
The only reason you laugh is the because you've been brought up to believe the old racist "drop of blood crap" that was at the core of the *spit* Confederate slave holder *spit* ideology. When the disgusting trade in human flesh started in North America there was a serious problem. The filth engaged in it enjoyed raping their victims.
There's nothing quite like tail that's absolutely powerless to stop you and can be tortured or killed at your whim. I happen to think it's repugnant, but a lot of guys seem to have been excited by the idea. The problem is that these women sometimes got pregnant which left a troubling problem. What do you do with the babies? You couldn't think of them as human beings. Why, that would mean that a darky's sprog was just as good as a White Child and might inherit or use its legal status to get itself and its mother out of slavery. Heaven forfend.
The solution was straightforward. White women having sex with Black men became the ultimate betrayal of the Race. It was unthinkable which led to an awful lot of Strange Fruit hanging from trees later on and the subject of an awful lot of porn to this day. In much the same way the insatiable Jew-slut was a staple of Nazi stroke books. White men having sex with Black women was fine as long as it was done discreetly.
The babies were dealt with by the simple expedient of saying that the tiniest bit of African genetic material made a person Black. Hence the "drop of blood" name. In some places this led to complicated systems of mulattoes, quadroons, "high yellow" and so on. But basically, the point was to ensure the purity of the White Race by denying the humanity or at least the privilege of being White to anyone with a trace of slave ancestry.
In South America it was different. White, Black and Indian are much looser and more defined by social roles than skin color with most people considering themselves vaguely mixed. I have a friend who spent eight years in Brazil doing Capoeira. He is paler than I am which is quite an accomplishment. But because of the kind of Portuguese he spoke, the job he had, his friends, neighborhood and the way he dressed he was Black by local standards.
In Muslim countries the important thing was Islam. The children of a Muslim slave owner and his non-Muslim slave were generally raised as Muslims and freed because of it.
There's just as much justification in saying that Obama is a White man with a Black father as saying that he's a Black man with a White mother. Your perception is the result of a very specific time in American history and no more obvious or natural than any other primitive taboo.