What could cause a Volvo 940 to exhaust Black smoke?
Black smoke is normally an indicator of a very rich fuel mixture. The 940 was known to have problems with the fuel pressure regulator. Also while you are looking around under the hood, check for any of the hoses from the turbo charger and the intercooler for holes. Especially the one at the intercooler on the drivers side. They tend to rot out on the bottom causing the boost pressure to be to low to handle the amount of fuel being injected.
If it is a turbo, the turbo needs attention. Bearings? Oil leak? Worse?
It could also be cauesd by the Brain box and air flow meter. So, you also need to make check-ups on those parts.
Turbo chargers that have shaft/bearing problems blow light blue almost white oil smoke, so you can pretty much rule out the turbo itself. The "Brain box" has never been known to cause this on any 940 series Volvo, that pretty much puts it out of the situation for now. Mass air flow sensors/mass meters on turbo 940s usually cause a lean mixture problem not the "black smoke" rich mixture unless the lining of the air filter housing has came apart and the air passage through the mass air flow sensor is blocked. This would give the same problems as the hoses having holes or leaks in them that I spoke of in my first answer about low boost pressure. One very unusual time I experienced a very defective oxygen sensor to cause black smoke. That can be easily checked by connecting a volt meter to the single wire lead of the oxygen sensor and look for a voltage swing from .1 to 1.0 volts. The green wire coming from the computer supplies .5 volts of reference voltage so the sensor moves plus and minus of that voltage as it reads the content of the exhast before it passes through the catalytic convertor. Please don't forget to montior the fuel pressure! In psi it should be around 43. In bars it should be around 2.9. This is of course just my thoughts based on what I have experienced over my past 30 years at a Volvo dealer. Hope this helps.