If someone has one black parent and one white parent, why do they refer to themselves as black?

To me a person that has a white parent and black parent most of the time the person has more white in them (DNA) most black people have 20 percent of white blood...so in reality a person of white-black is white in majority. Like me I just got my DNA results from 23andMe and I am 68 percent white, 29 percent African, 1.3 percent Arabic and 0.7 percent Native American. For me if your less than 30 percent of something than you cannot just claim that, majority rules.