[Johnbot]

I tried:

For the roman emperor prompt, Bard produced:

17 white men

2 white women

1 black man

1 black woman

For your prompt, Bard produced:

21 black men*

Interestingly, for the Roman Emperor prompt, Bard never refused to produce an image, though once instead of an image it only produced alt-text for two images, and once it only produced a single image, while for the African Oba prompt, three times it insisted it could not produce an image of that, once it explained that it is incapable of producing images, and once it produced only a single image rather than a pair.

*After typing most of this reply, I went to run more to see if it would ever behave differently, and on the 22nd image it produced an image of a black woman.