You don't seem to understand how someone who is "only one quarter Black" can say she's black. You also don't seem to understand how someone can simultaneously claim membership in two different ethnic groups.
First, the sexual exploitation/rape of enslaved African women at the hands of white men began at the very beginning of the slave trade. It was systemic and long-lasting. Consequently, there have always been mixed-race black people living in "The New World." Because those mixed-race children were not allowed to marry white people, they bought their European ancestry into the black community with them and passed it on to their children. For that reason, just about every black person you've ever met in your life has some European ancestry. DNA testing of African-Americans bears this out. People like Harris, people who aren't 100% black, are the rule, NOT the exception, in the black community.
Second, the white men who fathered these mixed-race children felt that enslaving their own children in order to increase the number of slaves in the marketplace was morally acceptable. So those slave owning white men decided to implement the "one-drop rule", a set of laws and social customs that stated that anyone who had any percentage of African ancestry regardless of amount, was classified as black. This not only increased the number of slave, it also solidified white privilege by narrowing the definition of whiteness. There is not, nor has there ever been, a blood quantum to establish blackness.
Third, while the "one-drop rule" is no longer codified into law, the physical appearance of people plays a big role in how other people classify them. While Kamala Harris is very lightskinned, she isn't light enough to pass as white. And her natural hair texture (curly) hints at African ancestry. Newsflash! She looks black and she is perceived as black, and, given the fact that she actually has black ancestry, she has every right to call herself black.
Four, because of our changing social mores, American society is now more open to the possibility that people can simultaneously belong to two or more groups. Even the census now allows people to check multiple boxes. Kamala Harris' decision to claim both races isn't "wrong."
I don't care if you're confused or angry that you don't get to change the rules about racial identification. It is what it is.
And, no, Senator Harris didn't sleep her way to the top!