While acknowledging that the Roman Republic/Empire was complex and long-lived enough to provide us with dozens of questionable parallels for any scenario, I think we can agree that they are always neat.
We do not know for certain whether or not Septimius Severus was black. That is itself significant. One historian writing three hundred years after his lifetime claimed that he was ‘dark’, and one or two portrait statues appear to show him with African features. But the vast majority of images that survive make him look like any other Roman emperor before him – his whiteness over-emphasised by the shiny white marble in which he was so often portrayed. This was not a black man claiming the imperial throne for himself. This was the Roman imperial machine turning a man of colour into an emperor more or less indistinguishable from all his predecessors. The machine was making sure that race did not show.