Originally Posted by AndyOne
When running a tuned pipe some of the burnt fuel is forced back into the cylinder just before the exhaust port closes due to a high pressure pulse. This recirculation of burnt fuel causes it to be burnt even more on the next firing stroke. The result is black exhaust oil, it's just an artifact of using a pipe and nothing to worry about if there's no sparkly bits in it.
Thanks for the replies folks.
I'm inclined to "run" with the reply from "AndyOne"