You know how Jack Black imitates instrumentals with his voice, like "brig digga dig" and all that stuff?
When someone does that, it is called 'Jack Blacking it'.
Popular among enthusiastic metalheads.
Steve: "DUN DUN DUN- wa nanana, nanana, nanana! DUN DUN DUN- wa nanana, nanana, nanana!"
Dave: "What is it with Steve and 'Raining Blood'?"
Bob: "I'm not sure but he's really Jack Blacking it!"