Cold air is drawn in at the bottom of the BB units is warmed and exits the top of the BB units and flows up the walls. The warm moist air hits the cooler wall and condenses on the wall leaving air born particles as a stain. Most BB heater covers have a thin adjustable deflector the can dirt the air flow. If they are fully open the warm air goes directly up the wall if they are closed some it will direct the air out away from the wall a little. Try closing the deflectors a little and see if that helps.
The BB heaters should be kept clean and if the water temp were high enough to burn the dust chances are your boiler would have gone through the roof by now. JMHO
Be sure you live your life, because you are a long time dead.-Scottish Proverb