I’m aware that race is not a strictly defined category, and that racial classification is arbitrary to a large extent. However, this doesn’t answer my question.

It demonstrates the logical fallacy - if it's an arbitrary classification, as you say, how can those blood test groupings be accurate? They can, at best, be arbitrary

“Arbitrary” doesn’t mean “uniformly random”. Even though the lines are blurred, there is still a strong correlation between being considered part of a particular racial classification in the US and having certain genes.

Thanks for the interesting first link. Now I understand your earlier comment