Black smoke is an indication of too much fuel, thus poor mileage. You might have a bad injector, which would cause one plug to become carboned up. Have you checked all of them? If one is black and sooty you likely have a bad injector. If they're all black the problem is something that affects the whole system. A fuel pressure regulator will sometimes leak into the vacuum line and the engine pulls fuel that way. A sticking regulator can cause the pressure to be too high. Regulators and injectors can be expensive so proceed carefully. And that's just the mechanical side, various sensors could be making the computer think it needs more fuel. Vehicle computers of that vintage were not as smart as they are now. Sensors had to be waaay out of range before they will set a code and turn your light on.

I'm sorry I don't have the magic bullet, but hope something here helps.