Oh, just thought of a problem with casting a Black man as Luthor.
Bald. Lex is bald. White men generally do not shave their head, except perhaps those who are naturally balding. Black men more frequently do it for fashion. This is probably because of the incorrect notions about "skinheads" and the negative connotations there.
But Lex is bald, usually because of failed science of some kind. Sometimes caused by Superman somehow. Him being bald is a tragedy for him and it brands Lex as "weird" looking. But a Black man being bald won't have the same impact. Culturally speaking, it's not nearly so unusual to see a young Black man with a clean-shaved head. A Lex who is bald but looks "normal" doesn't give the same feeling as the unusual-looking Lex who probably would never have chosen to be bald... and Lex would never shave his head just because of a receding hairline like some middle-aged men might.
And going with a Black actor would ruin the possibilities of seeing the wild red hair in one way or another. Flashbacks to childhood? Cloned body? And what of Con-El? Should he be made from Superman and Luthor's DNA, wouldn't that result in a "mixed" Superboy? That might be even more unnerving for middle America than the mere idea of Superman and Luthor essentially fathering a child together.