FWIK that should be a color shift that takes place over time based on the bluing salts and the metal alloy mix, I'm not an expert but perhaps when that part was blued, the blueing salt bath concentration as well as the temperature of the bath were too low.
You see it in a good number of vintage guns, SIGs and some lugers included, but the few Krieghoffs that I've seen never had that shade.
IMHO
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