Originally Posted by Zeph

Mine does exactly the same. I presume it's the same thing you get with cars when you start them up and get water plus soot sprayed out the end of the exhaust. My bike did it when it was in standard trim and is exactly the same with a DNA stage 1 and 2 and the "Kev" mod set full on.
Maybe we should both try wipping the deposits off once the engine is hot and see if any more arrives. That would confirm the cold start water spray theory.