Doesn't pure-blooded just mean "this group of humans hasn't intermarried with other groups much since the not-so-recent past"? How is that nonsense? And why li

Doesn't pure-blooded just mean "this group of humans hasn't intermarried with other groups much since the not-so-recent past"? How is that nonsense? And why limit it to humans? What makes us so different from other animals in this regard?