The hot tank black oxide process:
5 lbs. sodium hydroxide (lye)
2.5 lbs. ammonium nitrate (can use ammonium nitrate fertilizer)
1 gal. water (distilled is best)
Heat to 285 deg F in a mild steel container (do not use stainless, it contaminates the solution and gives a bad finish) and immerse objects in hot solution for enough time to get the color you want. You likely will have to bring the solution temperature up but that happens automatically as the water is boiled out of solution.
Safety issues: this hot solution is VERY caustic and will eat skin if it gets on you and you don't wash it off immediately with cold water. Wear good eye protection as you won't have time to wash it out of your eyes if any gets there.
The following was added by DD:
Wear face shield, rubber gloves and rubber apron when you work with that stuff. Don't wear good clothing. Leather shoes will be eaten by splashes and dripping of this stuff.
Try to maintain the temperature at 285. You need a thermometer. Temperature may vary at higher altitudes. You may need to add water or let water boil off to control temperature. To add water slowly trickle it down a metal spoon into the solution. Don't pour straight in. You will get a steam explosion.
That solution will also eat concrete, sheetrock, and wood. The high temperature steam from the solution will permeate everything and needs vented. If not vented it will rot and rust everything.
You need to completely degrease the metal before putting it in the blueing solution, best done in a boiling degreaser solution. After blueing completely rinse all the solution. this is best done is a in a trickler or flush tank where water flow in one end of the tank and out the other.
One other thing to consider is take your finished gun down to you local gunsmith and ask him if he does his own blueing. If he does ask him what he will charge for dipping only, no polish or assembly. If all he has to do is dip and he is smart, he will be reasonable.
I use to take in blueing at the hourly rate for an aerospace company that brought stuff into my shop . All I had to do was load the stuff in the baskets and move them from tank to tank. Later I just sent the baskets back with the finished stuff and they brought them back loaded. Made some good money doing that and it helped keep the bills paid.
