Why all the black stuff in the barrel?

I am new to PCP airguns and cleaned the barrel in my Diana Skyhawk today after putting approximately 300 pellets through it. I was amazed at how BLACK the cleaning patches were. I would expect that kind of filth in a powder burner, but not a PCP rifle. Is this normal? Is it just lead residue? Is there something I should be concerned about? Is it possible that dirty oil from my Yong Heng compressor works its way into the reservoir and then into the barrel?
 
A lilte off topic but lead pencils have never been made of lead:) If you do a search on the internet, you will find the reason why it is called lead pencil.

"In the 16th century, a large deposit of pure, solid graphite was discovered in Borrowdale, England. ... When metallurgists first encountered this substance, they thought it was some sort of black lead, rather than a form of carbon. Thus, they called it “plumbago”, which is derived from “plumbum”, which is Latin for “lead"