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.
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.
An alien taxonomist, perhaps.
We humans go "bug! whale! snake! cow!" most of the time even for species found here on our own planet.
We would even name the sub-groups firstly the artifacts that differentiate them physically.
Blue Whale, Grey Whale
Black Bear, Brown Bear.
As we develop a better understanding, we'd also probably later determine that the genetic differences may not map very well to the physical attributes ... but due to historical groupings based on visual cues, we'd continue to overstate/understate the differences in the textbooks and in pop culture.
They'll presumably have had things like https://en.wikipedia.org/wiki/Carcinisation happen on their own world.
I wonder about that... I can't tell the difference between, say, a male octopus and a female octopus, but octopi can. Maybe the differences that seem so obvious to us are actually almost imperceptible outside of our species.
Whatever categorization they came up with would most certainly not resemble what we consider racial categories today.
'Aliens' visiting Earth would immediately categorize us into groups very crudely resembling the groupings we use today, because our visible characteristics are the most immediately obvious artifacts of our existence.
(Edit: when I say 'immediately' I'm indicating this would be an obvious, first order thing to do from the first pictures they have of us, not an 'Enlightened Alien' scientific form of categorization).
If all they had were 'pictures' of us, the race categories we use would be the obvious grouping, or something resembling that.
They would see that most of the people in Sub Saharan Africa looked quite different from those in East Asia. (And difference between Sub-Saharan Africans and East Asians is more than 'melanin').
There's a 'continuum' between every biological grouping, that doesn't mean those categories don't exist. It just means we're going to argue a lot about where and how to draw the lines.
Race as a 'Social Construct' relates to all of the other attributes that we associate with race, and individual lived experiences due to how they are perceived etc..
To your point, Aliens wouldn't immediately pick up on the 'Social Construct' bit, at least not right away and so they wouldn't have the prejudices that we do, but if they could only observe from afar, they would see exactly what we see, and visual distinctions would be the 'first order of separation' even if it was, after further understanding (i.e. genetics) a less important distinction as you hint.
Edit: someone provided this like I'd like to also include it [1] which illustrates some of the current debate over the notion of race, and that it's clearly politicized.
[1] https://en.wikipedia.org/wiki/Race_(human_categorization)