What bothers you the most, the fact he was black or the fact he had a big cock and the idea made you feel inferior? Talk with her about it and clear the air. WI. She feel like it would be a one time thing or is it something she would like to continue? Then figure out if you can live with her desires either way. Would you like to do a revenge fuck and tell her about it? Where do you want to be in a year?
Depending on how jealous you are, you might want to talk to her and ask her how she might feel about a mom threesome with him, her and you. She might like it, and he may feel less threatened by knowing you accept and even will take part. Talk it over with her before you do anything.
this idea/fantasy that black men are "hung like a horse" is pure myth. multiple studies have shown that there is not much difference between the penis size of an average black man and an average white man. there is no biological reason why the penis size in one race of humans should be noticeably different than another race. but it makes for a compelling fantasy.