If its a Diesel they are supposed to Blow black smoke. They burn a gas and kerosene mix of diesel. However the check engine light doesn't mean you ruined your car even if its intermittent. 99% of the time when the check engine light comes on it means one of the sensors that the ECM monitors has gone bad or is out of range. As in 02 sensor, speed sensor, map sensor, maf sensor, idle speed sensor, there are probably 20. There's even a coolant sensor. If any are bad the engine light comes on. It can shut off and come back on intermittent. It doesn't come on if your engine valves are bad or a bad piston. They are codes the ECM (car computer) throws out letting you know you have a drivability issue. I had my battery temperature sensor go bad. I've done my own work on cars with check engine issues. Get it on a diagnostic machine for $40 before you do anything else. A bad sensor will make the car run rich and make black smoke come out with a rough idle. Especially the mass airflow sensor.

If the engine light is not on, it should be fine to drive. Obviously it is not burning as much diesel as is being injected. That can be caused by a number of things.

Black smoke is over/unburnt fuel not timing. Have you checked the air filter lately?

" Just wondering if I should be worried driving it to the dealer, "

You do realise the engine light being on means there is a problem and the reason it is now playing up is because you chose to ignore it. You could be facing a big bill now where as if you got it repaired as soon as the light came on it could have been a cheap repair. I doubt driving it would cause any more damage than you have already done.

Black smoke from diesel-Timing is advanced

2003 Jetta TDI. Engine feels sluggish at low rpms, at higher rpms I'm getting a LOT more power then I'm used to, and it chugs black smoke out. Engine light has been on and off all winter but it's been off for the last two months. It's not due for an oil change. 235,000km. Just wondering if I should be worried driving it to the dealer, or if I should get CAA to tow it.