"Of course, it could be argued that America's success is due to all those black and brown people that have enriched our genetic stock."

If you were a racist, you might make that argument. And anyone who was a racist should see it the way you mention above.
The nazis if they actually believed that the existence of the Jews could be their problem, should also be able to see the Jews as part of the reason why they weren't still some backward country.

This assume racism has any validity- which it doesn't.

Racism as "practiced" depends upon a materialistic ideology- that humans are biological machines.

Another "foundation" of racism is the value assigned to one's blood. That an American would adopt an Asian child and require that child learn about Asian culture and have that culture be more significant for the child instead of the American culture is demonstrating too much significant to "blood".
That one restrict adoption of black or white children to only be adopted by corresponding black or white parents, this is too much emphasis on blood [or simply racist policy].

What is significant about humans is human culture, rather than biology. I would say it's in ballpark of +90% of human is software, than compared to hardware. And in term of social policy, it should have nothing whatsoever to do with biology. Nor do I see biology as having any positive or negative affect in terms of how well a nation does.