One of the common cause is a clogged air filter, a clogged air filter will restrict the amount of air coming inside the engine combustion chamber. Therefore, when a black smoke comes out the exhaust of your car the first thing you need to check is the air filter, check whether the filter is clogged. When doing this just remove the air filter and visually check if there an accumulation of dirt on the filter. If dirt is present then it is more likely the cause of air/fuel ratio being rich. Fixing this one is easy you just have to replace the filter.
But for instance, there is no dirt present on the filter, then a possible cause of the problem has something to do with your car fuel injection system, the fuel injector sensor may be bad, the job of the sensors is to measure various engine factors that use to calculate how much fuel is injected into the engine cylinder, if the sensors gone bad, the air/fuel ratio will be wrong causing the black smoke out the exhaust. The sensor that commonly causes black smoke from exhaust when faulty are the mass airflow sensor, throttle position sensor and engine coolant temperature sensor.
It is difficult to identify which of the three is the problem therefore you need to bring the car to a reputable car shop then ask the mechanic to check your car with engine analyzer, replace the sensor at fault to to fix the problem.
Fuel injectors may be leaking as well.
