What primers are you using? i found cci's to be the cleanest when it comes to blowback.

Try some glass cleaner to get it off, i sometimes have to scrape the fouling out of my flintlock, lock,with a screw driver too buti guess thats something flintlock shooters have to live with. After you get that stuff off, put a thin coat of oil in that area and you can just spray and wipe the fouling away next time. I use to use 3in1 oil on my renegade and coat the sidelock, rear part of the barrel and stock with it to make the fouling wipe off easily.