I have never ever seen,in all my years as a small engine mechanic specialist ,working only on outdoor power equipment.a lean running engine produce a black plug.In that case they are grey/white looking.
Sorry to clarify no a lean engine doesn't turn the plug black BUT if someone inexperienced is looking at an obviously discoloured plug just gets told if it's black its running rich, they look at the plug and see it's grey they may take it as being black. this is why I asked for the photo. Not very well explained but for someone new to reading plugs it can be hard to tell black from grey with no comparison.
Pete
