Take the distributor cap off. For each plug contact in the cap clean with very fine sandpaper or emery cloth, so there is no oxidation on the metal contacts. On the rotor, clean the metal contact also.
Carbon fouled
Black, dry soot on the electrodes and insulator tip indicates a carbon-fouled plug. This can be caused by a dirty air filter, excessive driving at low speeds, too rich of a fuel/air mixture or idling your vehicle for too long.