Hi Dark1961. In my experience when you clean the bore of most springer type airguns you will see a dark residue on the cleaning patch (or whatever cleaning medium you use).
Often this is made up of residue off the pellet, or the burning of lube from the spring or piston. This is quite common, and generally speaking is nothing to worry about.
If the amount of residue is significant it could indicate that the gun has too much lube on the internal workings, or there is wear that is allowing the lube to reach the face of the piston seal. A good sign of this issue is that you will often see smoke coming from the bore when you fire the gun.
There are a lot of opinions about the need to clean the bore on an airgun, that are worth you taking time out to read. I hope that this explanation assists. Richardo.