I like the Dulite.  With the right polish, it produces a bluer blue than the black produced by Brownells.  I'm certainly no pro though.  Bob

Not sure what is being blued; but nitre bluing would be a bit challenging to do large parts such as a rifle  barrel.
And since there was no mention of it, a huge word of caution when using molten Saltpeter.
Chemically, it is an oxidizer; hence it's value as a component in traditional black powder.
When heated to a molten state, you are at approximately 600 degrees; and anything, and I mean ANYTHING that contains carbon that may fall into this liquid, or if the liquid would splatter out on a wood work bench or floor will create a serious situation.
I know whereof I speak !
Years ago, I put saltpeter in a small Lee electric lead melter to try bluing screws etc.
It worked well enough; but not wanting to keep the saltpeter in the melter, I decided to pour the molten material out on the ground.
Unfortunately, there was dead grass in that area.
As soon as the molten saltpeter hit the ground,it immediately began a sort of roiling and boiling and smoking like a huge stink bomb. It eventually burned itself out; but not until it had consumed every bit of combustible material within a 2-3 foot circle !