you will always have a slight rich result on an aircooled engine, mainly because the plug heat range is quite cold and secondly the engine is relying on moving air over the fins for cooling, why tax it heavier by leaning it out.
Best would be to check the airfilter and give it a thorough cleaning, install back the plug, and do some WOT throttle bursts, overrun to slow speed, lug engine at 20% throttle till power falls off about 5 or 6 times then kill the engine dead, pull in the clutch and coast to stop - check plug immediately - you will have your fuelling condition for your idle mixture and slow working.
The idle mixture only affects till 10-20% throttle opening combined with the idle jet size. After that its needle taper, emulsion tube and main jet progessively. Do not start to fiddle with the mixture like our charya roadside mechanics do. And by the looks of it the plug looks pretty ok to me, brownish tinge on the shell - if the shell and ceramic were sooty (powder) black - then it would be called rich.
The thing that is questionable is why are the plug threads black, seems to be living a protuding life - is this plug actually the correct plug for your engine?