>>7662326Blame the casting choices. Originally they had Chyna be the voice of Dr. Facilier's daughter Freddie in the animated series, but they wanted Chyna to have a big role in the second movie so they had her play Uma in the live action instead.
The 3rd movie isn't out yet but they had a different girl play the live action equivalent of Freddie and I have no idea if she's a better or worse choice for her.
Uma might also be black in the Descendants books but I haven't read through them yet so I could't tell you.