"Fertilizer bluing" is a well known process.
As said, the metal prep is the secret, it must be of the texture you want and must be completely degreased.
When you go to dunk it in solution, bear in mind that you are working with boiling caustic, which will attack flesh faster than acid. Wear protective gear.
Also the Dunlap formula releases a lot of ammonia. Work outside.
You can get similar results without the fumes by substituting sodium nitrate.
Note that all the sodium hydroxide does is to raise the boiling point of the solution to where the nitrate will oxidize the surface of the steel, it does not play a part in the chemistry of the bluing.
Full description at
I cannot find Ken M.'s description of how he does it, but the approach is similar except he puts up with the ammonia. Says it quits after a while when the solution is well mixed and reacted.