There are times when I press the thimble in place and the only black that transfers is along the border.
This works for me. With a sharp chisel sideways carefully scrape the black away or go straight down with the chisel flat side to the work. This is done by hand with a bench chisel...no mallet, the chisel needs to be sharp.
Do you trim the borders until no more black transfers? Or do you want to see that black?
Only if it needs it. You have to think when you use the black. It's very easy to become a black removing automaton. Tight sides will show black,,,that's what you want. If it has already moved past there's no need to remove the black.
Example......
Your rod slides easily into the the entry thimble but the tang is still proud on the fore stock, the inlet shows even black. If you remove all the black the tang will still be high. To bring the tang of thimble down, only remove the black from half the pipe back since the front is where it should be.
Do you trim the borders until no more black transfers? Or do you want to see that black?
If you remove all the black, eventually the stock will be gone.
If you leave the black around the edge, what do you remove if that is the only place that transferred?
If you are high and have black on the edge and nowhere else, That's what's holding you up so it needs to be removed. Round thimbles will mark very thin lines on the edge like that.
I am getting nervous about trimming away wood here because that is where the erros will show making the inlet look sloppy.
On the Gillespie I took the rails of the ramrod channel down to reveal 3/4 or more of the ramrod. A good bit of my careful pipe inlets wound up as dust on the floor.
Always think ahead....The lock inlet was great on that rifle when the wood was high.....when I took the wood down little gaps opened up. It worked out but that's why you need to be neat and true even beneath the surface. The surface now....likely will be shavings and dust on the floor after shaping.
A lot of this is just doing. It's a skill set that takes time and the only way to really improve is to do it and learn from mistakes and success..