Even though King Arthur is mythological and may not have existed, it would be inappropriate to make a film and cast him as a Black actor
As an Asian who lives in Asia, all this race-swapping in American Hollywood is really weird.
I don't care if Super Man or Spider Man are Black or if 007 is a woman. They exist in the modern world where heroes and villains come in all races and types. And generally, comedy and comics should be taken as tongue-and-cheek.
But whether it's A Song of Ice And Fire or Greek mythology, (neither of which are historical), the casting is inconsistent with the setting.
It would be dishonest to say that A Song of Ice and Fire is not based on Eurasia. And every region in the fictional world is a stand-in for the real world. The Norse. Catholics. North Africans. Byzantines. Mongols. Indians. In such a world, it would be pretty fucking weird if there were Asians integrated in the North. Or shirtless Scandinavians raiding with Dothraki.
Fictional worlds aside. Greek mythology is very much cultural. It's a story of Greeks and their historical neighbors who would've occupied Turkey, the Middle East, and North Africa. There are plenty of tan-skinned ethnicities in this area of the world. Casting a Black actress is a far reach outside the boundaries of normalcy.
What even is the goal of this race-swapping? Does it promote national harmony to make decisions you know will stir up criticism?