Black stuff from defroster

I have a 1983 300sd. Recently the car has started blowing black "stuff" out of the front defroster vents. The material that collects on the windshield is as fine and black as graphite and now I'm getting larger (lint size) pieces collecting on the dash.

Last night it snowed in our area and when I first turned on the defroster it looked like I was getting some very fine snow blowing into the car.

Defroster blows hot air and does its' job after normal warm up time.

Can't find any previous post on this phemomina. What's happening and how do I fix it?