What gains did black people achieve during Reconstruction even though it was an overall failure?
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.
Black people during the reconstruction era were able to finally move away from the plantations. One of the first things that many of them did was to go around and locate their family members. They were able to reunite with many of them but not all. Once they were together they joined other black people and developed communities. The families became strong with the people all working together for a common goal. Many black people were able to find ways to obtain land and build homes on it.
Schools were built to accommodate their children's needs. Black people went off to school and later college. Trade schools were developed by black people to teach others trade so that they would be able to get work.