Black smoke is unburned fuel carboning up. A diesel needs the right amount of fuel as well as unrestricted air both in and out to burn cleanly. If the air intake is clear I would check the exhaust system for blockages and restrictions, especially the elbow where the water is injected into the exhaust. If the previous owner ran the engine at low rpm for a long time as you say it could be very carboned up.
Living aboard in Victoria Harbour