Blue Wonder is also a nice bluing agent.

Its a "warm" blue vs a hot salts blue or a rust blue. I refinished a cylinder on a Ruger Blackhawk this way and it came out looking rather nice. (holds its color still, after several cleaning sessions with Hoppes #9)

I've known guys who will build their own "swamp box" in there garage and slow rust blue old barrels in the middle of summer.

Rust blue tends to hold up better than a hot salts blue...