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Does anyone know what would be causing the residue on the one side of this spark plug? Please forgive my newbiness.

Oil

The side electrode throws a shadow over the plug if it s on the flame propogation side of the cylinder.
When you build racing engines you pack washers under the plug to get the electrode pointing towards the back of the head.

If you stopped the engine by turning off the fuel the dark area is what the plug would have looked like all over if you turned the engine off electrically.

Now if you killed the motor with the kill switch, then that engine is running lean or the plug is too hot.