Jim a couple of questions.First where did this happen?The second would be when was it found?
The reason i ask is simple.2 years ago i found quite the same thing here.The bare ground appeared to be blackened.The grounding conductors within the panel for about 3 maybe 4 ins had the same discoloration.Ok these did not have a gen set on them but it was a cause of concern.Other than discoloration there was no real cause for concern.
But it was puzzling.At first i attributed it to a residual lightning strike.But this occured during a non lightning time of the year also.This was in IVY Lakes off of SR 54.I did meet with Paul C CEI of pasco and what he told me was that they believed that it was due to a chemical reaction within the soil.Since the poco had direct burial and conduit into the meter base.They thought that there was a chemical reaction within the soil that sent a chemical cloud into the meter can and consequentially into the panel through the nipple and panel.
The rational was that this only occured on homes that had been trimmed out and with a meter blank and panel cover.So the possible chemical fumes permiated the enclosures and discolored the wires.
I disputed this since the actual bare #4 running down the exterior wall was also discolored.He took a wire sample but never heard anything of it again.You might want to contact him in RE: to what they found.Just a thought.