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Hot bluing is hot, but rarely over 310°F which shouldn't be hot enough to affect the temper of any modern steel.
https://en.wikipedia.org/wiki/File:Tempering_standards_used_in_blacksmithing.JPG
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Looks to me like it was "hot blued" like a gun.

I remember when I was working in a gun shop and we did hot bluing, that you could get a nearly black finish if you left it in the tank long enough....