Replace the Fuel pressure regulator and see if you still get black smoke first.
I vaguely recall there is a way to measure O2 sensor with a multimeter, normal O2 sensor has 3 pins, one is common ground, one is the heater power: once you start the car, it heats the O2 sensor to let it reach functional temperature faster, the last one is the O2 volume indicator.
You can let the car run 10 minutes or so to let O2 sensor warm up, then probe the voltage from the O2 volume indicator between the common ground, can't recall what exactly the voltage should be, maybe something around 800 mV.
Actually for the fuel pressure regulator, you can also measure it first, I am pretty sure there is place in the fuel rail where you can attach a cheap fuel pressure gauge and measure the fuel pressure, but since fuel pressure regulator is cheap, you can just go ahead to replace it.
Keep us informed.
99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.