Jeep Cherokee 88

Black smoke generally means that your fuel mixture is too rich, which makes sense if your gas mileage is that bad - burning more gas per mile travelled.
Have you had a decent tune up in the recent past? Air filter could be clogged/dirty, Timing off, Plugs fouled, dirty carb or fuel injectors, while you're at it check the distributor cap for cracks, burned terminals inside, check condition of rotor. Just a good tune-up would be a good starting point. If problem still exists, post back - an Oil Company spy may have reprogrammed your engine computer in the middle of the night, telling it to run rich so you'll have to buy more gas $$$$ - Nahhhhh! Good Luck!