I used several acidic applications, i.e., onion and lemon juice, and a ketchup/mustard mixture to achieve a sort of color case hardening effect.
I get that effect using mustard only, by purposely applying it unevenly. Looks great.
If I just want an even layer of patina, soaking the blade in a 50/50 mixture of vinegar and water seems to work pretty well. (Be sure to remove any oils, fingerprints, etc. from the blade, beforehand.)