When it sets at idle it is fine. But when I start to back up the driveway it starts missing real bad and blows black smoke.
Black smoke is an indication of fuel that did not get burned. The engine miss may have something to do with this. The best approach would be to check for any existing check engine light codes that may have been stored. These codes can point us in the right direction. Fortunately the 1993 Pontiac is an OBD I system. This means we are able to pull the Pontiac Codes without a scan tool. Once you have pulled the codes post them below in the comments for more information.