These are fantastic questions!!
There was definitely a free Black elite in Charleston. They were often skilled artisans, relatively light-skinned (they'd call themselves the "free colored" or "free brown" elite and are now rolling over in their graves at me calling them "free Black," actually), mostly of mixed racial ancestry. But their presence doesn't mean that there wasn't also a free Black middle class, and a free Black poor. The laws declaring the presence of free Blacks illegal were just not really operative most of the time. Occasionally they could provide reasoning to expel someone under exceptional circumstances, but most of the time they didn't have a huge impact on people's lives. Free Black artisans could serve both black and white clients; in trades like barbering, there basically wasn't competition from white tradesmen and so whites all went to black barbers. In other areas, white tradesmen complained all the time about being undercut by black tradesmen. I think there are a lot of parallels between undocumented migrants today and free blacks in the antebellum South.
The best books on this topic are by George Reid Andrews. The first is The Afro-Argentines of Buenos Aires. The second, and the one that really gets to this question, is Blackness in the White Nation: A History of Afro-Uruguay. He basically argues that beginning in the mid-19th century, censuses begin to systematically undercount African-descended people in Argentina and Uruguay. Censuses begin to re-categorize light-skinned African-descended people, and eventually these distinctions collapse into whiteness. But I highly recommend Blackness in the White Nation, it will answer a lot of your questions!