I use both Slow Rust (Cold Rust) bluing and Express Rust (Quick Rust) bluing.
I used to use the Belgian Blue formula (Quick/Express Rust) and some others in that same catagory. They worked great. Mostly due to the mercury compound(s) in them (merc bi-chloride(?).
The old formula Birchwood Casey muzzle loader Bbl Brown was another that worked very well. Also contained Merc BiChloride.
(Their improved formula does not)

After many years of feeling bullet proof from such things as merc poisoning/damage to the nerves, old age finaly rings a bell and tells you maybe there's a different way to go,,just to be safe.
Yes we all played with liquid merc in school 'plating' copper pennys and such and none of us died from it AFAIK.
But the constant exposure and more so the thought of the merc in the carded dust,,on the tools, the floor, all over the shop and myself and clothes told me it's likely not a very good plan for the future. Even if I don't care about just me, how about others around me.

So I went with Mark Lee's Express Blue maybe 25+ yrs ago and am entirely satisfied with that for the Express Rust process.
I admit that I rarely use the Express Rust Blue anymore except on some touch up and hurry up jobs. But it certainly has it's place. Lots of kitchen stove top small jobs, it's great for that.
No merc in it so I am fine with the stuff.

Brownells brought back the old Herter's Belgian Blue a some yrs back and said it was the same orig formula. That old stuff had the mercury bi-chloride in it.
I wondered if the new Browneels formula maybe substituted something for the Mercury.
I bought a small bottle of it and tried the test for merc (swab some on warmed Brass, see if the mercury 'plates' out onto the surface).
It did,,I was surprised. I would have thought that selling a mercury compound soln would be off the table at this point in PC/Greeny time.

For Slow Rust Blue I use Laurel Mtn Forge rust soln. It has bit of nitric acid in it as well as copper sulfate.
I've used a lot of other solns as well,,this one works well and I've stuck with it overcoming it's tendency to after-rust and to
'plate-out' the copper at times when applying.
I used to use Sal-Ammoniac water soln way back. Some chips of the soft solder flux disolved in pain water
Most anything that will cause a fine rust will work.

I try not to let the Slow Rusting cycle build up very much,,just a very faint brown/red color on the surface. No heavy rust coating
I lightly drag my finger tips down the side of the bbls, if I can feel a very slight roughness,,they are ready for the boiling water tank.
You will get a fine layer of color with just a faint rust coating.
No need for the rust coating that looks like an old piece of farm equip left out in a field for a few years.
That just pits the metal and muddys up the water tank.