Let's see, do you know the difference between a vaginal orgasm and a clitoral orgasm? If he leaves your wife in tact (read does not remove the clit...) I'm pretty sure you can still give her an orgasm.
A black mans cock as you put it is a bit of a vague description. If your wife is tight and ends up with a black man with a cock that will stretch her she might not be able to feel you penetrate her as much for a while. If she has given birth via her vagina it's very unlikely there will be much of a difference.
In any case there is no such thing as stretched beyond your width. If you wife can cum now, she can cum from you in the future via vaginal stimulation. Even if she fucks with black bulls that stretch her. She might think your cock is not as much fun and therefore be turned down by it as a result but that would be a psychological thing. Some wives might actually like the feeling of their white husband cock not touching their walls as much.