Hot blueing is the process used to “blue” firearms. This involves submerging the steel parts into either a boiling solution of potassium nitrate, sodium hydroxide or a mixture of nitrates and chromates. Alternatively cold blueing can be used but the finish is not nearly as robust and it is hard, or impossible, to get a good finish. It is only really meant to be used to touch up scratches.

There is a interesting discussion over at rec.guns about cold bluing an entire barrel and receiver.

Well, I just completed my first bluing project using Birchwood-Casey’s gell
and boy am I disappointed. I had a nice, shiny preped firearm and ended up
with a streaky one -barrel *and* receiver. Man, I can’t figure out what I
did wrong but it ain’t pretty. I slathered on a coat of gun oil to protect
it afterward and it looks like what you get when you lay on an uneven
coating of oil but it’s in the bluing. Any ideas on what went wrong will be
greatly appreciated. I’m glad that this isn’t a good shotgun so I guess
anything is better than the rusted hulk that I was given.

Personally if I wanted to re-blue a firearm I would get a gunsmith to hot blue it.

Read the thread here.