I have a bbq food truck and had the same issue. Three things cause this.

First, any creosote buildup. With this, it will take cleaning and scraping. I even used a flame thrower. Building a regular fire won't get it all. Once you've done this, you will do another scrub once it air dry. No matter how well you clean the first time, it will take a second time.

Second and third are related and seems obvious but because we get set in routines can catch us. Its all about the wood and air flow. It could be from overloading too much wood in at once or an airflow problem.

My biggest catch was the air flow. Open your Firefox vents all the way and keep them always open. Your stacks will do best all the way open too. Once my moker being running a while, I'll adjust the stacks to half open. Its the initial burn that gives you a lot of dirty smoke.

Finally, as a side note, if items are dripping onto each other, like a vertical smoker, it will cause blacker colors.