bhai jan, yeh engine weak ka kya matlab hai? engine toot jaye ga?
I think the blackness is probably due to un-burnt carbon which comes out of the exhaust from time to time or something similar...like you heat a piece of metal using candle and it turns black if it is slightly away from the flame. similarly the engine shouldn't turn black because it is very close to the flame. I mean the flame is inside it, so its closer than anything.
almost all exhaust systems have that blackness after some time of use, from what I see.