You mean the white edge around your subject, right? you could add more corrections, selecting different parts of the black area since you don’t have uniform black, and only make small changes to each. The edge might be a little smoother that way.
However, you will probably have to live with some halo effect, especially if you’re using compressed footage. Its similar to the issues of getting clean edges when keying, at some point the edges just aren’t smooth enough.
You could also try to mask your subject with SliceX from CoreMelt . It uses mocha tracking so you don’t have to rotoscope manually if your subject is moving. Simply make a mask and soften the edges, looks better than a white halo. Or if you don’t want to spend any money you could try DaVinci Resolve Lite. Its more complicated but you can get the hang of it quickly and the integrated tracker is very good.
“Always look on the bright side of life” – Monty Python