It's not obvious to me that the cartoon character is unambiguously black rather than brown. Berbers and other North African groups that were well-represented in the Roman army have brown skin. In any case, Beard points to instances of Romans in Britain who were either described as having black skin ("Ethiopean") in the historical record, or whose skeletal morpholgy suggests Sub Saharan ancestry.

Also, nothing is "well known" about the skin color of populations thousands of years ago, for obvious reasons.