Usually, historians point to the fact that blacks managed to set up stable communities of their own outside of the context of slavery.  This is not a big deal in terms of politics or of rights, but it is seen as evidence that black ex-slaves were not helpless.

For example, one of the books I have used to teach from talks about how the free blacks managed to set up their own churches and fraternal societies.  It also emphasizes that blacks were able to "reestablish and reaffirm" the families that had been broken up by slavery.