Black Smoke Marine 3126 engine

Good Day All.  I have a problem ONLY at start up.  First the boat is based here in So. Fl so outside temp is not the problem.  Only during the first start up of the day the port engine blows black smoke for about 15 seconds and then for the remainder of the day no smoke.  RPM is great with no drop in RPM either at idle or under power.  Fuel filters are clean as is fuel.  i realize this could be an injector problem but mechanic is not sure and wants to go into engine.  Any thoughts or suggestions?  Thanks, Capt. bob