I'm not sure what you're trying to say here. Are you saying a Roman in Africa didn't notice the guys around him had black skin?
>For example, in the colonial era, how was it decided in Louisiana if one man was black or not? The answer is, less than 1/32 of his ancestry must be of "black blood"[0]. This sort of thinking and arbitrary ruling is how race was established as a social construct.
This is non-responsive. You're talking about the legal response to race, which is irrelevant to the question of whether race actually exists.
In fact, you've written nothing that addresses the point. When a white guy and a black guy are walking down the street, do you have trouble when someone asks you to point out the white guy?