Well, for what it's worth, Memory Alpha states:
"In the teleplay, however, only Lutan's guards were specifically written as being African. It was director Russ Mayberry's idea to make all the planet's occupants African. Disgusted by this decision and Mayberry's attitude towards the performers, Gene Roddenberry fired Mayberry late in production. The remainder of the episode was directed by an uncredited Les Landau."
Apparently the source for that information is Wil Wheaton's review of the episode
. But Wheaton himself presents it as hearsay:
I've read that the Ligonians were not explicitly described as entirely African American in the script, but were cast that way at the behest of director Russ Mayberry, who apparently went on to be so offensively racist and treated the actors so poorly that Gene fired him before the episode was completed and handed the directing responsibilities over to then – First AD Les Landau. [Citation Needed] (Ironically, Mayberry went on to direct quite a few episodes of "In the Heat of the Night," which proves either that he learned something from this experience or that he's really good at directing stories about racists.)
So it's certainly a story that's widely propagated on the Internet, but it doesn't seem to be proven. And it wouldn't be out of character for Roddenberry to try to pin a mistake on someone else after the fact by making up a story about them.