From a social PoV, in Cuba, someone as you describe 3/4 one and 1/4 the other would be likely be put in the category of the 3/4. In some places an albino is just someone with different genetic characteristic, in other places they are a creature that brings bad luck and suffer violence.
Dog and cat breeds are distinguished throughout the world. It's genetic and socially constructed as well.
Race is definitely drawn along genetics lines - you've just demonstrated that in your example.
The 1/4 black - 3/4 white person was not identified as 'Asian' in your example, but rather, in the mixed physical scenario was crudely categorized as one or the other, in the example you gave, Black.
(FYI I tend to disagree a bit about the category though: I believe people will be categorized mostly for how they actually look, not so much the 'ratio' of anything. There's a lot of 1/2 Filipino 1/2 White people on TikTok who 'look' 100% White and make funny videos about the fact nobody believes them about their heritage).
Your example shows that race is definitely a social construct, but that it also has underlying genetic realities.
Further features that seem very important to us like the shape of our eyes or skin tone may seem irrelevant to a creature which doesn’t have a face and sees the world in a different color spectrum than we do. They might group based on smell or habitat marking groups of urban, suburban, and country dwellers.