Sure, I guess Tiana from Disney's The Princess and the Frog is the first African-American Disney princess... considering the fact that she and Pocahontas are the only ones who are American at all!
But Kida (Kidagakash) from Atlantis: The Lost Empire is a black princess, too - just not one that Disney likes to promote, for one reason or another. (Is it because of the resmblances to the anime Nadia the Secret of Blue Water?)
Personally, I consider Atlantis a highly underrated film. Then again, I do have a soft spot for steampunk, ensemble casts, Michael J. Fox, dorky linguists, conlangs and Mike Mignola. It's like a Disney animated version of Stargate, and for that I love it.
But for all the good PR Disney received for having a black princess in Princess & the Frog, you would think they would want to play up the characters of color that they already have!
Granted, Disney have a very questionable past as far as race is concerned, from Peter Pan's 'What Makes the Red Man Red?' to Lady and the Tramp's cringe-inducing Siamese cats. But Atlantis has a surprisingly diverse cast, including several well-rounded and entertaining female characters. It's definitely a forgotten gem in the Disney canon, to my mind.