Unusual Etch on a Moro Blade

[MichaelZWilliamson]

I realize this is a very old thread.

Mild acid followed by boiling water is one of the techniques for cold rust bluing, typically used for double shotguns. Acid, boiling water, wire brushing, repeat 8-10 times will give you a deep, blue-black finish.

So the vinegar followed by hot water came close to almost performing the first step.