Sometimes what people see is the oil they used to protect their bore. Especially if the rifle has sat for a while and that oil has accumulated dust & dirt. But a "dirty" patch simply means you have to pay more attention to the method you clean the bore of the rifle. The oil you use to protect the bore will also weep out fowling. So that usually means the barrel was not as clean as it could have been, the protection you used, drew more of the fowling, and now you're finally removing it. Its a lot more common to find this with traditional rifles then inlines, although if you clean them right, normally you're fine. A lot of people over look the bolster and the very cone of the breech when they clean traditional rifles.