either...
1. you have a rich map setup on the ECU, if it's stock and your running a "performance" map then you'll flood the engine.
2. you have an exhaust leak and/or the o2 sensor is going out.

if your running a gutted cat or no cat then the engine will naturally blow black smoke. it's unburnt fuel in the exhaust stream due to there being no back pressure on the o2 sensor and it's telling the ECU that the engine is running lean and the ECU compensates by dumping more fuel in.

most likely though it's your ECU.