And my question would be, What are these dark particles and where are they coming from? If it is soot, that indicates a leak in the burner vent piping. If soot is leaking, then the real concern is Carbon Monoxide is leaking also. CARBON MONOXIDE KILLS!!!!!
If you don’t have a carbon monoxide detector, you need to get one and put it near the radiator soon. If it was my house, I’d be on the way to the store now, not typing on this ‘puter!