Black smoke is unburned fuel: the engine is being given more fuel than it can burn with the air and load it has. The fault is almost never the fuel system itself, and adjusting fuel delivery to chase the smoke usually makes things worse.
Work through the causes in this order. An overloaded or fouled propeller is the most common — if the engine cannot reach its rated maximum rpm at full throttle, it is overloaded, and everything else you check is secondary. A dirty hull adds the same load. Then air: a blocked air filter, a restricted engine-room supply, or a turbo or intercooler fault starves combustion. Then exhaust restriction, which traps what the engine is trying to expel.
Injector and pump faults do cause black smoke, but they sit at the end of the list, and confirming them needs test equipment. Establish maximum attainable rpm first — that single measurement separates an overloaded boat from a sick engine.