Do a compression test first, after confirming that the valve adjustment on that cylinder hasn't tightened up for some reason. If the plug is black and oily, you've either got a really bad valve guide, or the oil is getting past the piston rings on that cylinder. Dry, black soot indicates the plug isn't firing for some reason- - - -shorted plug, bad wire, even one flat lobe on a distributor cam can kill a single cylinder.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!