And let me give a shout out to Mark Lee and his Express Rust Bluing solution. It is just the best.
I can remember 35 years or so ago stopping in to Mark's shop in the basement of Ken's Metal Finishing and seeing all the test bar Mark was testing his various formulations of his solutions on. His diligence paid off in spades (Black/Blue ones smile ). The bluing process was, rather then being a drudgery, a very enjoyable and near fool proof process. So easy, much simpler then the various video's on YouTube would want you to believe. Piece of cake IMHO