I assume you mean direct slavery, and yes it did occasionally happen but for the most part the Union Army looming around made keeping slaves as actual slaves pretty risky. There are accounts of it taking some places a significant time before informing the slaves, but this was by far not the norm by any means.

Rather, Southerners kept blacks enslaved via "Black Codes" and share cropping, which was slavery in all but name. These policies extended and got worse after Reconstruction ended. Essentially blacks became enslaved to their land and perpetually indebted to a plantation owner rather than being enslaved to a human owner.

EDIT: It is worth noting that poor whites also became trapped by share cropping and actually became de facto slaves. Much of the violent racism towards blacks post-war was from whites who saw their plight as caused by blacks freedom. Therefore the rich in the South channeled and guided that hatred towards blacks and not towards them.