That might be leaking from the valve cover gasket onto the outside of the plug. Got a V-6? Do they have those plug-tube seals like the I-4 engines? Basically in the I-4 engines the plug is down in a hole thru the valve cover. There's round gaskets around that tube that like to leak oil down onto the outside of the spark plug.

I'm looking at the brown stain towards the bottom of the ceramic part. Maybe evidence of arcing? That's one reason to put plug-boot grease in the rubber boot at the bottom of the coilpac.

The plug doesn't look bad but the gap looks big. What's the gap supposed to be for your car??