In the Spartacus series there are some African actors playing gladiators, especially Peter Mensah as Oenomaus (Batiatus' doctore). He has done a great job but my question is, were African people that common in Rome in that era?
Gladiator fights have a long cultural and political history. Originally they were held on a smaller scale (than the Colosseum) and often for the purpose of honoring a deceased. Their popularity grew as the rich began to see how they could further their standing with the people.
The gladiators themselves were often killed or severely injured, making it less suited as a long-term career for normal, free people (although there were free men doing it to rapidly rise on the social ladder). Most gladiators were slaves, forced to fight to win their right to life. As Rome had a lot of military success, a lot of slaves were either captured soldiers or just foreign prisoners. Rome waged war in most of Europe and around the Mediterranean sea, giving it a steady supply of prisoners.
At the time, the notion of 'race' wasn't as prominent as it later became (during American slavery for example). Slaves were branded by their originating countries and less on their ancestry. Being African wasn't a problem in ancient Rome as they were accustomed to and respected foreign African traders.
For slaves, this meant that their skin tone was less important than their originating countries and their numbers in the capital was more related to the most recent war. The only reasons for a higher number of African slaves are genetics and perhaps the exotic taste of the Romans. As the slaves were allowed to have children, which were then born into slavery, their genetic features could have slightly skewed the demographics.
All in all, the slaves ethnicity was more used to denote their origin and enhance their stereotypical capacities. The barbaric tribes (north of Rome) were typically associated with their physical strength while Greeks were preferred for their cultural refinement (acquired in their life before enslavement). Africans were typically more used for household tasks rather than for their physical strength but again this is only a stereotype and an individual assessment was made.
The Slave Systems of Greek and Roman Antiquity gives a lot of background information on slavery,
The Roman slave supply gives some numbers on the slave population and the means of acquiring them.
This link talks a bit more on ethnicity and related sources