i just pulled my head of my car and my piston head have black on them that i cant really rub off any ansers to why this might be it used to have nos but i didnt really use it much

 

probably just carbon buildup. are there any signs of detonation?

 

well i would belive the nitrous would of made it burn hot enough to burn the carbon out.... Kent makes a spray that is called carbon and gasket remover if you can find that (I have it at work at a ford dealership) and anyways spray that stuff on and wait a minute and the carbon is lifting off but break clean will work alright too. change your oil after. why it might be? running rich, not opening it up enough, is it burnt oil? could be alot of things

 

i have no clue it used to burn a little bit of oil but i had a couple bad valve seals