I would think that the blackened plug could be indicating a rich mixture rather than oil fouling. If it's sooty then it means too much fuel, only if it's wet and oily does it mean oil problems.
If you have sufficient oiling to wet a plug, then your exhaust gasses will be smokey.
However, maybe you are taxiing with the engine too slow. At low rpm the cylinders suck harder against the closed throttle. So there is more suction to pull oil past the rings and/or valve guides.
.
Last edited by scifi; 21st Sep 2016 at 08:19.