Was the engine warmed up or had you just started it? Black smoke is caused by incomplete burning of fuel caused by a rich air/fuel mixture. A cold engine requires more fuel to run. Black smoke usually means that the engine is putting in more gas that it is supposed to. This could be a number of problems and quite often is hard to pin-point and repair. Sometimes it is something simple, but usually not. If this is the case, and the engine has been driven for a while in this condition, the catalytic converter may be clogged up and need to be replaced which is expensive.