The darkened part may be hard to remove. I'd suggest a tiny out of the way spot.
Chromium over 12% (or 11?) is considered the main factor in most modern stainless steel; it is also considered to be a promoter of crackiness, so it's OK for knives, but not for swords. The properties of metals within alloys can be considerably different than when they're on their own. For instance, either the chromium or the iron by itself would react with the bluing solution, but combined as stainless steel....
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