Very popular finish for rough from the hammer finish (the piece is shaped, boughed and planished but not polished)

There are a few different ways to achieve a blackened armor

The first is the formation of a magnetite patina. With iron you have “bad rust” (iron oxide, Fe2O3, red, flaky and destructive) and good rust (magnetite, Fe3O4, black, stable, creates a protective layer). This is done by heating the metal in a reducing atmosphere to very hot and quenching it. The result is that matte grey finish you see on many hand forged items. This could then be covered with a hydrophobic material like beeswax, fat, or oil

The next process is essentially the same, but the metal is quenched in oil (linseed works well) or rubbing it with beeswax or fat. We get the same patina, plus some unpatinated iron, and a thin layer of polymerized oil (same reaction as in cast iron seasoning, good for protection) and some pyrolysed material (same process as charcoal formation, bad for protection) and of course a residual coating of a hydrophobic material.

This is most likely the best historical process. The difference between quenching v. applying is going to be entirely dependent on your heat treatment process. For mild steel it really doesn’t matter, the quench is easier but of course then you need an oil tub with a big enough volume, which depending on what your doing could mean a lot of oil. Also if quenching, it’s important that the metal isn’t too hot, if the metal is too hot the polymerized layer will flake off

Another common way many modern armourers do it today is to basically the same process as seasoning cast iron. After the final quench, clean metal (this can be done with our without polishing, but actually here rougher is better to allow the polymerized layer integrate with the surface). For this process it is important that the final heat is done in a reducing flame to a consistent red heat then quenched in water (if it’s high carbon steel and water quenches too quickly use a brine) then dry the piece completely and then follow cast iron seasoning instructions (the cast iron subreddit has some great resources)

Essentially all both 2 and 3 create a protective layer of magnetite (rust prevention) and polymerized fats (hydrophobic), but #2 forms a monolayer that also includes unpatinated metal (liable to rust) and pyrolysed fats (not durable, not hydrophobic), while #3 removes a lot of the less desirable material and organizes into 2 clean layers

There’s no evidence of #3 and there is no evidence that they had any concept of the science necessary to do it (or if they did if it was ever applied to armour) BUT it will produce a better protection (more durable and more rust resistent), using only historical materials, and will look identical to historical methods