Maid Marian was like King Richard's nephew though wasn't she? She's totally almost sorta kinda a princess.
As for the first black princess being a chambermaid, most of Disney's Princesses don't start their stories out being royalty. Cinderella being the primary example. Snow White and Sleeping Beauty spend more time in cottages than castles. Belle was just a silly girl who reads. And as mentioned, Tink and Mulan were never royalty. Jasmine and Ariel are really the only "Princesses" (Well and Nala but as she's a non-anthropomorphic animal princess she doesn't count
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