Sounds like an air leak somewhere....hole in fuel line,loose carb? I'm guessing that if it was running way too lean because of an air leak it might cause the blacker then usual residue and I would also guess its possible for the residue to pushed from the pressure line from the muffler back to inside the tank. Makes me wonder after a flight just how crappy the left over fuel is inside my tank.....even on a good running flight.

Just watch because even if you got it running again,you might not notice an air leak until its put under the same flying conditions. I've noticed that if you notice your engine not wanting to come down to idle rpms,theres a good chance your leaking air thats makes your engine run fast(lean). And the emptier your tank gets the harder the engine has to suck for fuel and thats when the leak may show its face.....perhaps moreso during a straight up climb.

Test you engine by only putting minimal fuel in the tank and hold the nose of the plane straight up in the air at full throttle for awhile, like maybe close to 10 seconds or more,enough for the fuel thats already in the line to be used up and create that vaccum affect from the carb. Make sure your tank plug is tight and not leaking too and that the clunk lines etc. inside the tank are good.

Let us know how you make out!