Nobody has made that claim.
Sure, some racists have the same idea of what ‘black’ means. And some, perhaps even most might even agree to a certain degree. But that doesn’t mean everybody does. I’m reasonably sure that, even among Americans “blackness” is not always solidly defined, and would probably range from the “one drop” rule to a hardcore racist, to “having dark skin” to the average person. Heck, from my experience in places like Austin, some people with dark skin are classified by those they meet simply as “ethnic”, and most people have no idea what “race” they’d actually belong to. But all of that doesn’t mean that race has an actual operational definition, or any real predictive or descriptive power in most of its uses.
The point is, as well, even if a large amount of people agree on what makes a race, the liminalities blur in any case and there can be (and generally are) disagreements about what constitutes the main body, anyways. If we asked 1000 Americans, probably 1000 would say that Chinese people were Asians. But how many would say that all Pacific Islanders were? How about Hawaiians? Likewise, I’m sure you’ve encountered at least one person who’s called any random Hispanic person “Mexican”, a name that would be considered as quite an insult to many people from the numerous countries, other than Mexico, that’re south of the border.
It also doesn’t mean that continuing to act as if they were valid categories which actually and adequately separated humanity into discrete and useful groups, is at all helpful.
As we can all agree, race is a biologically useless term. And if it has no actual scientific validity, then it is only a socially constructed term, and one that changes from time to time and person to person, to boot.
And if it is only socially constructed, and has traditionally led to great suffering and injustice… why can’t we change or eliminate its construction? No, not today, and not by fiat, but over time and by replacing it with a more accurate view?
There are, and have been, a great number of social constructs that time has done away with, from “the weaker sex” to “white man’s burden”.
Why can’t traditional fallacious concepts of race join 'em on the bonfire?
The problem with this, and your earlier formulation that discrimination based on race isn’t racism if it’s “good” discrimination, is that the definitions lose any objective meaning at all.
For instance, discrimination, while having a connotation of being negative, does not have that denotation. Nor, in all cases, would discrimination for a certain group be spared from a racist label. If a business decided “we really prefer white people, so we’re going to discriminate for white people”, that’d still be classified as a racist policy. Correctly.
Would a dirt-poor white family, who can’t afford to buy shoes for their children and have no power at all, not be racist for hating “those damn niggers?” They’ve got no power, after all. I hope that you wouldn’t use the answer that just because they’re white, even if they don’t have any actual power, they have power. That way is treating any white person as fungible. Or if you prefer “Awww, they’re all the same.”
When Farrakhan goes on about how he hates Jews, or whites, he’s not being racist? I’ve often heard the claim that “a black man can’t be a racist”, and it boggles the mind. What… if a white man says “fuck those darkies.” that’s racism, but when a black man says “fuck those white devils” it’s… not? What if, in the context in which it takes places, the black man has more ‘power’, however we’re going to define it, than the white man? Can a black man be racist on the streets of Harlem, for instance, but he becomes non-racist in the halls of Congress? If he’s the same man, with the same beliefs, does he really “become” anything based on where he’s sitting?