Polyglot, it is interesting that you said on the edge of blackness. This happens to be a portrait of a dark hair on pure black backdrop. I bounced some light off white ceiling from my flash to throw some separation light on her hair. Assuming that the cast was created during exposure or during film processing, how can I learn to detect this cast on film before printing.... my eyes dont have that level of expertise. and how to avoid next time. ??

So having said this, yes your burning/dodging explanation makes sense, I just need to practice it. and lots of paper may go to waste in the process...