It’s the difference between “Black” and “Colored.” If African genes joined your family tree within the last 400 years, you may be light or dark, but you’re a Black person.
No source but Africa can make you Black. Not Native America, Latin America, the Middle East, South Asia, or Polynesia. Lots of roots can give you pigment, but African genes are magically powerful.
The one-drop rule is a ratchet, not a bidirectional pipe. President Obama is 50% White, but he is Black.
Americans actually act like this is based on reality. Blackness is such a powerful state of being that the slightest hint of a taint appears and you’re at the Black table. Nobody ever calls Obama White.
I love the phrase “mixed race.” It’s the opposite of one-drop thinking. A mixed-race person is not one-drop Black. They are a confluence. The existence of “mixed race” thinking makes me hopeful.