I was at the range shooting an AK, one day a few years ago, and a guy was shooting competition-grade a flintlock rifle next to me. I got to talking to him (the initial black powder worm started boring into my brain), and when he wrapped up, he showed my how he cleaned his rifle.
He stuck a toothpick into the flash hole to plug it, and then he filled the bore with a mix of Simple Green and Ballistol and let it sit for a few minutes. After that, he pulled the toothpick and pushed a patched jag down the bore, pushing the gunk all out through the flash hole. From there, he followed it up with dry patches and then swabbed it out with Ballistol to protect the metal.
I did something similar with my Traditions Trapper, but I put a little Damn Good Bore Solvent in and followed it up with boiling water. I used padded pliers to hold the barrel because it was so hot. I then pushed a patched jag down the bore after pulling the toothpick, and everything came out nicely. I read a blog before I bought my first BP revolver, and he suggested only natural oils for BP fouling, so I used olive oil to oil up the bore after drying it out with a dry patch, using the downward force of it to push air out of the flash hole, ensuring that no moisture was left.
After my first trip, I did see a brown patina in the bottom of the barrel the next day, so I used a patch worm to spin an oiled patch down there to swab it up. After that, there were no more recurrences. My second (previous trip out), I used Frog Lube, which is also a natural oil. I got no patina after using it.