Nice work!

As long as the charcoal starts to consume itself (burns with a white ash),,it'll produce the results you need.
Allowing a draft to fan the coals and boost the temp will result in a grey color and then a heat scale on the the parts.

The deep blue/black color is produced at about 830/850*F.
That just happens to be the temp that wood charcoal burns and consumes itself w/o any further help from an outside heat source.
The occassional scrubb down is a burnishing technique to even out the color and remove any blemishes on the surface.


Scale won't form when it's out in the air being burnished down as the metal temp is below red heat,,or it should be!
(You can get the same blue/black color look if you Nitre Blue and run the temp of the salt bath up the same 830/850F temp.
A lot more dangerous than doing it this way though!)


Imagine doing this over a huge open hearth of burning charcoal all day and with hundreds of parts as Colt, S&W and most every firearms mfg did at one time.
The factorys used child labor to do the work in many instances as it was dangerous but didn't require a high degree of training or skill development over time.

Small parts handles make the job easier as you found so you can scrubb and quickly return the part to the coals. Bury the part and rake coals over the part again and move to the next one. Keep in rotation and you can set a pace with the number of parts you are doing and the size of your set up so you have no down time and the job gets done fairly quickly.
It's still hot work! But the results are so worth it.
Nothing like it for those early SxS trigger guards.

Most modern instructions warn NOT to use charcoal Briquettes as they contain a binder (clay?) and possibly other materials to spoil the finish. You seem to have proven otherwise!.
I've always used small chunk charcoal. Didn't seem to much matter what brand. Some say they use Aquarium charcoal.
I don't know,,It's much like any of the finishing processes. Lots of different ways to get results.
You find one way that works well for you and stick with it.