Coking in the combustion liner. It builds up, breaks loose and comes out as a puff of black smoke in the exhaust. You may see some evidence of this when you pull the combustion cover, and occasionally see some quite large build ups around the surround to the nozzle. Nothing to worry about, unless something else starts happening. Ive nevwr seen any damage from this - its a piece of real hot carbon going real fast through a real hot engine, followed by a nozzle and fast wheels.The comments on turbine lock up, carbon inspections in the sump, etc, are all important to watch, but not what you are seeing here. I've seen it in all variants of the C20 in every airframe.