actually, if the cylinder wasnt firing... the plug would be wet... not black... thats carbon from to much fuel.... sounds more like a leaking injector..try changing the injector to another cylinder... and see if THAT cylinder's spark plug is black after some driving...
have you checked for a header leak? I've had one that sounded like a rod knocking... was THAT bad....it's usually better to eliminate the simple/cheap stuff 1st..
