This question has no answer.

Race, as you and I both understand it, is an entirely social construct. It has no basis in any biological reality, and it stems from the past four to six centuries of social, political, and "scientific" thought. It is therefore impossible to map a modern understanding "race" onto the pre-modern (ca. 1500 AD) historical past.

A Roman would have told you that the Empire consisted of many different gentes, a word which can mean anything from "extended family" to "nation". A person's gens was extremely mutable (i.e. no "one drop" rule) and varied not only with generation, but also within generations based on social considerations.

Think of it like this: if I'm from Chicago but I live in Boston, when I'm at a conference in Austin do I introduce myself as being from Chicago or Boston? If I'm around some people from Chicago and I want to fit in, I'm going to introduce myself and say I'm from Chicago, and so too with Boston. We find this exact pattern with people in the Empire who define themselves (or are defined) as "Franks", "Germans", or "Romans" depending on who they're interacting with.

I can kind of here you thinking "yeah but they'd notice black people." Again, we need to return to the idea that race is an entirely social construct. You or I classify people by skin color (and a host of other features) into races, but there's no inherent reason why that needs to be the default understanding of what race is. There's nothing that stops people from defining "race" on hair color.

Just to emphasize one more time, race as you and I understand it is an entirely social construct.

Here's some reading on how the modern construction of race works:

• Hirschfeld, Lawrence A. Race in the Making: Cognition, Culture, and the Child’s Construction of Human Kinds. Learning, Development, and Conceptual Change. Cambridge: MIT Press, 1996.

• Hartigan, John. Race in the 21st Century: Ethnographic Approaches. Oxford: Oxford University Press, 2010.

• Van Ausdale, Debra, and Joe R. Feagin. The First R: How Children Learn Race and Racism. Lanham: Rowman and Littlefield, 2001.

• Baran, Michael. “‘Girl, You Are Not Morena. We Are Negras!’: Questioning the Concept of ‘Race’ in Southern Bahia, Brazil.” Ethos 3, no. 35 (n.d.).