I was going to put together a long post about how it's important to make sure we get a proper historical picture so that we don't simplify the matter too much - while still being aware the whole thing was pretty universally bad - but I think instead I'll just put together a very simple construction or two.
People in times past have been willing to put up with quite a lot of bad treatment because of the uncertainty of what a revolt or rebellion would bring (we can tell this by the fact societies with serfdom tended to, um, still exist thirty years after serfdom was introduced). But if their only options are a revolt ("probable death") or an existence of the kind that has been ascribed to most/all slaves in the antebellum south ("rape, torture, hard work and probable death") then we should expect most of them to choose revolt.
This is why the lot of slaves was probably not quite as bad as people have implied in this thread - because there is a whole ocean of possible "bad" between "slavery" and that level of bad. This does not imply or say that slaves were better off as slaves - it says that people in history have been willing to accept bad things rather than gamble on worse things. It means thinking about the people who were slaves as being people able to make an assessment (whether based on incomplete information or not) about their situation.
Similarly, if one examines what happened during Jim Crow and the like, there were several differences between the lot of an 1850s slave and a post-Reconstruction black man. They are not all positive.
The white people are the same kind of white people with the same kind of attitudes (in many cases, the same people full stop).
They are not any more likely to be punished for mistreating a black person.
They are no longer financially disincentivised from harming a black person.
The black person can arm themselves.
The black person can earn money and own property.
The black person is free and is not owned.
For most people those latter three are probably the ones that are more important. But for some they may not be.
The reason I think this is important in an alternate history context is that it helps to give an accurate portrayal of the South. The South isn't somewhere where every black man is being whipped or tortured - it's more insidious than that, it's somewhere where "troublemakers" are whipped and where other slaves feel grateful they are not being whipped (even as they still hate that they're property). It's somewhere people congratulate themselves on their "enlightened" attitudes while being so wrong to modern eyes it makes your stomach churn, it's somewhere where in Louisiana there were hundreds of black plantation owners and where at least a few black people acted as soldiers - and where there was a genuine operational/strategic benefit to be had for Lee's army that it could rely on a logistics train consisting largely of blacks (free or slave, it's a little hard to determine).
This is more correct than the other approach (or approaches - it should not need to be said at all that the south was not a place where there was "happiness in slavery". It was a place where the majority of people who were slaves considered their lot to be broadly acceptable compared to the risky, bloody alternative). It's also got more scope for interesting scenes and nuance.
I seem to have ended up with a long post anyway.
So, to completely diverge from that, here's something else - consider how the Confederacy wins in your TL, both on land and at sea, and consider what lessons they might draw from that including wrong ones. (e.g. if the CSA wins in an ATL where they win with Picketts Charge or something similar, to use that as an example, they might continue with the cult of cold steel well past the best-before date, whereas if they win by outflanking and destroying McClellan's Army of the Potomac they might focus on manoeuvre at the expense of modernization.) Similarly, consider what lessons the USA might learn. (earlier adoption of quick-firing weapons for the Picketts Charge one, or the need for a single massive army impossible to outflank for the outflanking one.)