Regarding Sub-Saharan Africans, there's no one region of space that appears to have been predominantly colonised by them. That doesn't mean there aren't such worlds, just that they didn't wind up dominating an area. New Capetown, in the Lyran Alliance, is a pretty obvious example of such a world being a South African expy.
Racial division isn't a huge factor in the Battletech universe. On individual worlds it may be an issue but largely black, white and yellow band together in the common cause of sticking it to "those assholes across the border". Culture is a far more significant division than colour. The FRR worlds are a mix of the original scandinavian colonists, settlers from the Lyran worlds incorporated by the Combine into the old Rasalhague District over the Succession Wars (this was from the Tamar region which was fairly multi-ethnic ) with an overlay of Japanese culture from the Combine.
The FRR situation depends on the era. There's a
wiki entry for the state. The 20 Year Update probably provides the best information on the short history of the FRR up to 3050.
From the mention of the WoB it seems likely you're talking about the Jihad era. The Rasalhague region had seceded from the Draconis Combine around 3036 - thirty years before the Jihad. Before this time it had been a contentious region - once an independent state that was conquered years ago. Most of the worlds had been Lyran worlds conquered by the Combine over the centuries. As a result the FRR had a significant pro-Lyran population, a significant pro-Draconis minority and a strong but shallow nationalism. The Combine was not only a former oppressor but also something of a bulwark against the Federated Commonwealth and later an ally against the Clans.
Any characters under 30 would have never known Combine rule but they'd probably have long memories of the Clans sweeping in and almost entirely conquering the Rasalhague region. Older characters would likely compare Clan rule to Combine rule.
ComStar and the Word of Blake would be viewed in a similarly fractured way. ComStar supported the formation of the FRR and after the Battle of Tukkayid (fought on a FRR world) ComStar troops were the bulwark of their defense against a further invasion. On the other hand, the Word of Blake formed from the same sect of ComStar that had originally supported the FRR and could be seen as another small state struggling to survive among the vastly larger successor states.
My one character from the FRR in a RP was ethnically mixed Japanese-Scandinavian, wore a samurai topknot and went mercenary after his homeworld was conquered by the Clans. This made him somewhat among Rasalhaguans as they had recent memories of mercenaries who fought for them during the secession exploiting loopholes in their contracts and occasionally leaving the rebels exposed.
The Inner Sphere's been a huge melting pot over the centuries. Don't be afraid to mix things up.