1996 Chevy Blazer rough idle

Do you have a check engine light? If not is it working (when ign. Is turned to on, light should illuminate).

This is most likely a sensor out of range, like coolant temp sensor for example.
Black smoke indicates a rich mixture and most sensors responsible for air/fuel will go to a default rich setting if bad.
This is done to prevent a lean mixture which can cause severe damage to engine.

If light is on get codes read at most autoparts stores for free.