I seldom use a brush and prefer to clean with a patch and solvent only. I use a combo of wipe out and butches on my 300 RUM Sendero. I apply wipe out and let it sit for 10 minutes then patch it out. I follow with a wet patch of Butches and let set for 2 minutes, follow this by another patch of butches pushed down the barrel and back twice. I repeat till it is clean. I use no more than 12-15 patches after shooting 20-25 pills. I believe like the others, the copper brush is breaking down , giving you the blues! Once you settle into your own technique, clean your barrel but don't scrub it to death.
Definately switch to a less agressive cleaner like Shooter's Choice/Kroil combo and see if problem resolves.
I also use a surefire light at the muzzle to check for fouling. The intensely white light make the copper fouling easy to see, especially if you park the rod and jag 2-3" from the muzzle and give light something to reflect off.
I have a rifle that with a barrel that after 3 shots has copper streaks almost hanging out the muzzle. I found that using Wipe-Out and the accelerator if I just let it sit over night it takes out everything. There is no copper, no powder fouling and no carbon left to see. I don't have a borescope but if I run anything else through the tube I get no results. So I think it's clean.