A few things:
1. St. Augustine is commemorated, IIRC, in both the English Holy Transfiguration Monastery Menaion and in the Simonopetra Synaxarion. Though both editions have tried to include a wide range of Orthodox saints, their editorial integrity is AFAIK as good as anything out there in the Orthodox world. That IMO should really settle the issue.
2. As for Northern Africans, the wikipedia page quotes genetic studies stating that the vast majority of the population of modern Tunisia - be they "Arab" or "Berber" have been found to be genetically similar. This means that most Tunisian "Arabs" are actually culturally Arab Muslim Berbers (if you want to make such a distinction - in fact, I'd imagine similar is true throughout most the Arab world, to include the Egyptians according to wikipedia). The Berbers have been acknowledged to be the predominant people in the area since the Greeks first wrote on the area.
This implies to me that North Africans during Roman times were, with exceptions for traders, soldiers and their descendants, and other migrants, mostly Latin-speaking, toga wearing Berbers. I would not expect them to look much like sub-saharan Africans. And yes, they would have been considered Romans - in particular, Augustine was a Roman citizen who apparently felt quite at home when he moved to Rome.
[that being said, I'd guess that availability of actors/extras vice any sort of historical inquiry is the primary factor in how the North Africans of this movie look. And as another interesting aside, the wikipedia "research" I did for this states that examination of Egyptian Pharaoh Rameses II's mummy reveals he was a red head]