black smoke is usually the car running rich, so hardly think it's blocked injectors
i'm no mechanic, but i'd start with the basic ...MAF, Timing, CAS
also check the spark plugs to identify any problems in any cylinders. a misfire in 1 cyl in a v6 is hardly noticable
could be a very slight seepage of engine oil to one spark plug causing weak sparking or unburnt fuel