Flavio, blackening can be two things:
1. when steel is etched with acid, it can turn black, especially if it has been tempered. Tempered portions tend to turn black in the presence of acid.
2. when steel is older, the oxidation can create a patina on a blade. This surface patina can turn a brown-black, which means it is stablized oxidation.
3. when rust is present and it becomes stabilized, it can turn black, again a sign of stabilization.
Hope this helps.
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