Brute de forge is done before hardening, but can't be added to a finished blade.
Antiqued finishes and acid etching can be done on a hardened blade, but usually require the handle being removed and resist painted on to protect any place you don't want eaten.
A dark patina can be done with mustard, lemon juice, vinegar, or other slow acid. This is an easy project. Look up "Mustard Etch."