Historically, it did take tremendous force of character, because acknowledging one's black relatives, when one was phenotypically Caucasian, meant giving up all fundamental liberty and self-actualization in an American past that was an even more rampantly racist society than the current one. There is one town, in Virginia, where this was so common it was named Passing, Virginia. No one will put their house up for sale if you move in next door. No one will shoot you on the sidewalk in a racist drive by attack. You will not be stopped by the police for driving a new car that is either presumed to be stolen or paid for with drug money.
Most mixed individuals are of other blends.
Perhaps they function in a saner environment, the specter of slavery not hanging over their heads. Then again, perhaps not. They have a mixed identity because they really have no other. They own their mixed identity because it is true . They claim their mixed identity because they are tired of a world that is getting away with patently stupid judgments about people based on facts not in evidence.