I've noticed this on the Volvo, it currently has a snapped front pipe, and on the rare occasions I have used it since It broke over the weekend I have noticed it pops and bangs a hell of a lot when backing off the throttle.
Could the black spots be unburned fuel?
It does pop flames if I change down at high RPM, and seemed to be running rich before the exhaust broke.
I'm not really bothered, more curious.