Ahh now i understand, something like a blue cast. I'd like to call it an image artifact.
Well its hard to tell without any additional information and testing.
Judging from its gradial characteristics, makes me feel like there is something heating up at that edge.
This leaves question why blue? It should be most sensitive/amplified, since semiconductors are not that good at seeing blue, even more if this is shot under incandescent light.
This can be tested by taking some RAW file wich shows this artifact and "amplifying" (boost contrast+brightness) other channel (Green for example), to see if the same pattern appears.
I just hope the one you sold your camera to was aware of this thing. Nothing i'd worry about, but this kind of things usually come some week or so before CCD explodes with flames and smoke

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Btw.. my english is no better either. I just try to express my toughts with full sentences and not worry too much. In the mean time "learning" from the spelling and grammar checker.
