Another black/oily #6 plug. Anyone figured out why it happens?

Leaking valve cover gasket? Why would the plug be black though?
Changed my sparks today, those had around 15k on them.
Car ran fine, I'm not losing any oil. In fact I'll be due for 7k oil change soon and I still have 1/4 of original oil left in the iDrive thingie Changed sparks only because I had plugs sitting in closet for months and was bored.