A hundred years ago, scientists theorized that heredity (or "blood") had a telling effect on human character, and that race did, too. These conclusions went together. Race couldn't have a telling effect on character, after all, if heredity didn't. The members of the KKK and other racist groups had the apparent backing of scientific theory.
As the 20th century developed, however, racism fell into disrepute. Hitler gave theories of racial genetic superiority a bad name, and blacks successfully portrayed themselves as the equals of whites. Scientists came to theorize (and progressives to insist) that genes and race didn't make that much difference at all. Racists couldn't live with the idea that heredity doesn't make a big difference in character because then race couldn't make a big difference either. So racists clung to the idea of heredity's power, and the idea that heredity had a telling effect on character came to be seen as racist.
The move to deny the power of heredity in shaping character eventually reached the point at which, in the mid-80s, I was taught in an undergraduate biology class that gender itself was a social construct, and that a boy given a sex change operation as an infant and raised as a girl would consider himself to be a normal girl. (The startling case study that supported this assertion turns out to be an instance of egregious scientific fraud.)
In the last two decades, however, scientists have discovered (or rediscovered) the power of genes on character. For example, consider identical twin men raised apart. If one is gay, there's a 50% chance that the other's gay, too. That's genetics, and it's not what the people who see gender as a social construct wanted scientists to discover. (This finding goes halfway to serving the gay agenda of equality, but that's another rant.)
Meanwhile, the idea of race has continued to lose ground in scientific circles. The racial categories that have been useful in dividing people against each other do not correspond well with actual genetic populations, and the broad genetic populations that supposedly compose races are too genetically diverse for the races to be meaningfully distinct from each other.
Race is out. Genetic influence on character is in (again). Unfortunately for the racists, they are now in stark contradiction to scientific theory. While racists claim to believe that genetics has a telling effect on character, they clearly use this belief as a justification rather than as an actual, logical premise. If genetics has a telling effect on character, then you can tell something about someone's genes by their character. But the racists want you only to pay attention to race, not character. The racists want to consider the successful, educated black to be the genetic inferior of the no-account white. But if genetics has a big say in character, then one's actual character ought to be a better measure of genetic endowments than one's race. By not accepting character as an indicator of genetic endowments, the racists are denying genetic determinism.
In fact, while the scientists of a hundred years ago were indeed racists, they did not ascribe universal genetic superiority to whites, as the KKK does. Scientists, for example, conjectured about the lower classes and the genetic failings of the poor. The mentally ill and impaired were sterilized not because they were dark-skinned but because they were judged to be genetically inferior.
The 20th century started with genetic scientists and white supremacists roughly in accord. The 21st century has started with the racists espousing the opposite of genetic determinism (claiming that individual whites are genetically superior to individual blacks regardless of their relative accomplishments). It's ironic to see racists denying genetic determinism because genetic determinism is necessary to make racism plausible.