If you are using a copper solvent the patches will come out blue as you have noticed. But also with the bronze brush which has copper in it will leave a black residue in the bore from the solvent if it is a used brush. I dislike the ammonia based copper solvents for personal reasons. I've found that Hoppe's Elite copper solvent is a good product for removing copper fouling. And with this you can swab the bore and let it set for a while without stinking up the room and giving it time to do its magic.