The alloy of the steel will have some say in what the final 'color' of the bluing result is.
As will what rusting soln you are using, the degree of polish you go too (grit), how long you let the steel rust and probably a dozen other small variables.
You can go crazy trying to get the same (color) results on different steel components.
A couple things I would do in your case:
As long as you are polishing to 400grit, that is a high polish for rust bluing as generally done, but can be useful in obtaining a blue color more than a black tone. But you have to avoid over rusting the surface which pits the surface, more commonly called matted surface. That is what causes the light to NOT reflect back off the surface and gives the dark, black color to the bluing.
To avoid that matted surface, let the rusting form in a less humid and heated environment.
No artificial temp and humidity needed.
Coat the parts and let them hang in the open air inside.
Even at temps as low as the mid to upper 60*F and humidity in the 70+% range,,the parts will rust though very slowly.
That is what you want.
Rusting times will be is days,,not a few hours.
I let the parts hang for at least 24hrs before I even think about they might be ready and they almost never are this time of yr.
I keep the house temp at 65*.
I usually takes 2 days at least to form a rust coating that will give a color coating when boiled. Sometimes more time is needed.
BUT, the advantage is that the rust coating formed is extremely fine and very hard to actually see. Just a barely seen reddish color to be seen with a flashlite shown onto the surface. If you very lightly drag your finger tip down the metal you should be able to feel the slight roughness of the rust coating.
That is all you need to form a color coating when boiled. No more is needed and any more time rusting than that is just going to pit the metal which you do not want.
All this is what the the high grade makers do to get the very high polish finish on the bbls of the SxS bbls w/rust bluing.
Next is the boiling.
Distilled water is good.
Place the parts in the boiling water and let the water continue to BOIL. Don't shut the boiling process down.
Let the parts boil for 10mins at least . 15 is good.
Then remove them from the water and shake the excess water off. Blow any droplets off and/or touch the droplets remaining with the tip of a twisted papertowel to draw them off the surfaces.
Do not wipe down the parts with anything...
Hang them and let them cool completely before carding.
Card with de-oiled steel wool.
Denim is OK if you are going for the rough finish with the matted look. But you want to remove all of the loosened debris from the surface AND burnish the color layer with the steel wool.
When you have finished carding,take a clean large paint brush and brush the part(s) down to further remove any loose carding debris (oxide and/or steel wood flecks) from the surfaces.
Any of that junk left on the surfaces will be caught up in the next application of rusting soln and will dry with it. It will cause tiny speck marks in the bluing color layer. Maybe not noticable to some especially if you are doing a rough matted look type of blue. But if a high polish, blue color look is the target, then the absolute cleaner the better.
Next reapply the next coating of rusting soln. Just dampen the swab. Don't wet it down and apply as that will disolve the color coating already on the part.
The less the better. It should dry in a few seconds.
Don't scrubb it in. Go over the surface once. Do not re-apply over areas that you have already coated already.
As you get closer to your final rusting soln coatings, you can start to cut the soln % with distilled water.
50/50,,then even less. This will slow the rusting down even more and will help retain the high polish and blue color even more.
Rehang the parts for a couple days and do it all over again.,,and again...
It's a 'Slow Rust' for sure.
