In all my muskets, all shot with real black powder, in no case, does bp residue get under the barrel and into the tang/lock areas. There is only cap residue outside the barrel. Since my guns are all glass bedded, I only remove the barrel once a year after the competition season is over for the year because the torque on the barrel bands and tang screw will affect accuracy. I've never seen any problems in that area thus far so that's mostly a preventative procedure.
My cleaning process for black powder- remove spent cap and squirt windex down the barrel till it runs out the nipple. Scrape breech area with breech scraper. Patch bore till dry and pretty clean. Repeat with ballistol till patches are clean. Remove nipple and wipe area clean and scrap any fouling off including hammer. Clean flash channel. Put nipple back in with just a touch of grease on the threads. Wipe down with oil inside and out. About 29 minutes and done.