There is absolutely no such thing as “black brushing” and no history behind it. There is a history of using white actors and actresses to play Black people, or people of color, because people of color were enslaved, segregated, or otherwise not “allowed” to be in film. There is no reason to get a white person to play a Black person, or person of color, when you can get an actual Black person or person of color. Ariel is not a person. She’s a make believe character. She is a mermaid for goodness sake. There are no mermaids in real life. What you said doesn’t even make sense.