Car idles hard

Black smoke is excess fuel being burnt, rough idle suggests you have a misfire issue which would give you excess fuel.
I would start with a fuel pressure test, a bad pressure regulator could cause a rich condition, poor idle and black smoke. Next you will want to look at the fuel trims when the smoke is actually happening. High trims could suggest a sensor input issue like a bad coolant temperature sender. Basically the ECM thinks the engine is running cold and supplies more fuel. Extra fuel give you the higher rpm and smoke as well.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

To look at the fuel trims and sensor data you need a scan tool that can show live data.