Yes, good point! I am definitely aware of Richard, and Cleo Laine doing it (as Alan pointed out).
I think I didn't go back that far because I feel like the precedent wasn't quite set by that. For example, the Witch in the post-broadway West End premiere production was Julia McKenzie.
I can't say when it started, but it does feel like after Vanessa Williams *originated* the 2002 revival as the Witch, almost every major production has cast a black actress as the Witch... and I also can't think of a single major production that didn't cast white actors as The Baker and Baker's Wife.
However I will note that the major acceptation is the Shakespeare in the Park revival, which had Donna Murphy as Witch, and Denis O'Hare and Amy Adams as B&BW. So it's not consistent. But it is nonetheless a common thing... and like I said, it's only because this revival is on it's 3rd or 4th round of casting leads and it is STILL white B&BW and black Witch. Cinderella at least has gone from Asian to Black to Asian to Hispanic. But there's something about the two everyman protagonists and the fierce/angry/wise magic woman that has Lear, and many other directors, stuck in this white and black vision of these characters.