Running real rich and when gets to 3000rpm blows black smoke

Check that the engine coolant temperature sensor (ECT) is working, if it's electrically 'open' (as if disconnected) then the motor will run rich. By now, the oxygen (O2) sensors are fouled, that will make it run rich too. There are other factors that could make it run rich, like a faulty fuel pressure regulator, but check those first two first. Here's a directory link for you: https://repairpal.com/nissan-repair-in-oklahoma-city-oklahoma

mass air flow meter,atmospheric/baro sensor and /or coolant temp sensor,o2 sensor, on the side of the pcm is a low high idle screw, you can adjust the idle speed and see if this makes a difference in the richness to help diagnose