There's absolutely no reason Black Panther can't be portrayed by a white actor. As we keep being told, it's only about casting the best actors they can.
The Black Panther comics, through their worldbuilding and specific social/political themes, make it abundantly clear the people are black. T'Challa being black is an actual relevant element of his character. You can't say the same for ones like Domino from X-Force for example who was portrayed by a black actress in Deadpool 2. As far as I'm aware, there were no noteworthy comics where Domino being white was a relevant factor.
A time fans could complain was The Dark Tower movie. Roland being white is a relevant factor in the second book of the series, so there was grounds for complaints about Idris Elba playing him. It didn't really matter though because A) the movie sucked, and also changed the story completely from the books, and B) Idris Elba's (and Matthew McConaughey) performance was the only good element of the movie. But if it had been a better film and a more faithful adaptation of the story, there would be actual grounds to complain about Elba being cast.
To be clear, for an example of a white actor playing a character some fans criticized the idea of: I didn't have an issue with a white actor playing Goku in a live-action movie. Fans can nitpick all they want, but Goku IS an alien, and in the anime/manga he did grow up isolated with only his "grandpa" with him so being a non-Japanese person wouldn't draw that much attention in the nation, and then once the worldbuilding in the manga/anime establishes all the unique intelligent creatures who live among the population like a talking cat and a talking pig, Goku being "white" would not draw any more attention than Piccolo or Tien walking around with the Z Fighters in public like when they go to the Tournament at the start of the Buu Saga. Like, sure they get some looks from a few civilians, but no one is making that big a deal about it. Hell, in Dragonball Super, one episode has Piccolo go with Chi-Chi to help her shop and carry some bags for her.
Hell, I remember when a few Japanese news anchors saw the photos of the Evolution cast in costume, they immediately recognized Goku and Piccolo, but they took a moment before they could recognize Chow-yun Fat was supposed to be Roshi. Yes, they got an Asian actor in the role, but because he was missing Roshi's beard, turtle shell, bald head, sunglasses, and cane, and Chow-yun was still quite younger than Roshi in appearance, all the news anchors could initially see was, "Asian guy in Hawaiian shirt".
But ultimately, the problem wasn't Justin Chatwin being white (but I certainly wouldn't have an issue with a Japanese person playing the role obviously. And obviously Roshi, Bulma, Krillin, etc. should definitely be Japanese actors/actresses, or at least all Asian if they can't find a good Japanese actor for a specific character, the country in the manga/anime is Japan and the manga/anime gives no indication that the human characters aren't native Japanese citizens. Tien MIGHT be Japanese, but IIRC Toriyama kind of flip-flopped on Tien's original home or if he's even human), the problem was that Chatwin sucked shit in his performance, and the script wrote the character all wrong. That wasn't Goku, and it had nothing to do with him being "white".