Ok, not to be a dickhead here, and forgive me if this has been addressed, but I went through all the immediate responses to this post and no one asked for sauce. I have no doubt it is true (we have it all), but this is one place that no one gets a free pass.
>BLUE METAL
The commonest metal treatment for firearms is Blueing.
Real Guns + Blue Steel and Walnut.
Blueing:
DuLite
Oxynate 7
Prussian
Nitre
Acid
t. smithy