Just to clarify, Lucius Quietus was AFAIK from modern-day Morocco and not from modern-day Mauretania. The name Mauretania has migrated southwards since then and modern-day Mauretania is indeed a pretty mixed place, with both caucasian and ....... populations, but anyway that's a whole other story...
Quietus' cavalry-men, as depicted in Trajan's column, look pretty "white" to me at least. As do other natives from the wider area and time-period that we speak of, such as Juba II. They would pass pretty comfortably as modern-day Moroccans or Berbers. When it comes to references of dark skin within the Roman Empire, that's AFAIK associated with the Eastern part of Rome's realm in Africa, not the Western part where Rome kept itself to the Mediterranean coast-line.
OTOH, I have to assume that when someone calls a specific person from antiquity a "full-blooded black African", then there must be a good reason for that, unless it's one of those "crack-pot" theories that circulate the Internet (not that there's a shortage of those as well). Is there any ancient source that describes him in such terms?