Historically, in the United States, the “one drop” rule applied, meaning if you had one drop of black blood you were considered black. This was written into law in some (perhaps many) states. If you had someone who was considered white and it could be proven they had even one black ancestor, no matter how far back, they could be declared black and lose such privileges as voting, property ownership, ability to get an education, restrictions on where they could live, who they could marry, what jobs they could hold, etc. Since there were a number of vale paled-skinned people of mixed heritage attempting to “pass” as white to gain the privileges of Caucasianhood at the time this was a real concern. It also contributed to the harshness with which racial mixing was treated in the past, especially regarding the combination of black man + white woman.
There’s a scene in the play Showboat (which may not always be portrayed in performance) where a young white man who is in a relationship with a black woman is about to be arrested and jailed for breaking the miscegenation laws (usually considered a felony) and evades capture by having his girl prick her finger and him swallowing a drop of her blood. At that point he tell the sherriff “I have black blood in me, so now I’m black and it’s OK”.
Another pernicious effect of the “one drop” rule back in slave days was the sale of some very pale-skinned women, usually for lustful purposes. The result was some women who appeared as white as the slave-owner’s daughters, being auctioned off. Just for the record - 1/2 black was a “mulatto”,1/4 black “quadroon”, 1/8 black “octroon”. But a person with 1/32, 1/64, or 1/128 African ancestry was still legally black and could still be someone’s property. This also meant, of course, that most of this person’s ancestory was white, and since whites and blacks could not marry in most areas, we’re talking about up to 5 or 6 generations of white owners impregnating young women whether the young women desired this or not. That’s an awful lot of rape. When you consider that there was no bar against a white owner (or his relatives) raping any slave he owned (in fact, legally, it wasn’t considered rape) in some instances this was incestuous rape. Pretty fucking sick if you ask me.
So, part of the hostility to race mixinig across the black/white divide comes from brutal history where people were jailed for showing affection for each other, black men wound up hung from trees and lightposts on the mere suspicion of touching of white woman, and the rape of black women by white men (even by a father or brother) went unpunished (even when the resulting baby was clearly too pale to be the product of a union of pure Africans). It shouldn’t be shocking we’re still having trouble with this when it wasn’t until the 1960s or 1970s that the race marriage laws in many states were rendered void. You don’t overcome this sort of cultural conditioning in just one or two generations (which is no excuse to NOT make the effort)
Oddly enough, while there is some feeling against racial mixing of other sorts (white/Asian, black/Asian, Native American/anything else) it seems to me that it’s the white/black combination that still generates the most hostility in the US.
What I find most peculiar (and extremely sad) is that some of the women I overhear talking about Halle Berry and Venessa Williams as being “too white” (as if they had any say in their parentage!!!) are frequently too pale-skinned themselves to be of pure African heritage. This “too white” verdict is apparently based on appearance, not their behavior and certainly not their own declared ethinicity. Ditto for those of African ancestory criticizing Tiger Woods for embracing all his ancestory, not just one part of it.
Richard Pryor (unquestionably considered African-American here in the States) once talked about going to Africa and asking someone what tribe he appeared to be from. He was told “Italian”. Which, I suppose, clearly illustrates just how relative the concept of “race” can be.
The real truth is that we’re all mutts if you go back far enough - there’s no such thing as a purebred human.