Do you mean it's contagious? It can be passed from spouse to spouse? Because if you're saying that their offspring will get it, that's genetically passed down from the black parent, not the parent who doesn't even have it. Or are you saying that the offspring of a mixed race couple will be entirely white, but still inherit diseases from the black parent? I don't think I'm following you. Could you clarify?
Sickle-cell anemia is recessive. So, if a black parent who is a carrier (one allele) has a child with a white person who does not carry the allele at all, half of their children will be carriers for the disease (note that they will not actually have SSA, which is fatal, since they do not have two alleles for it). Now, say one of these kids marries another kid who also carries the SSA allele through the same process. One quarter of their kids will get full-blown SSA.
When I say that with inter-marriage you'll eventually see white people with SSA, I really mean to say that you'll eventually see people who look white, but who have one or more black ancestors, with the disease. My point is that SSA is now almost excusively a disease found in black Americans (though, is there really such a thing as a "pure" black American? There was a lot more mixing of blood going on in America's history than a lot of people are willing to admit), but there is nothing preventing white, or Asian, or hispanic Americans from getting the disease.