The main thing going on here is the difference between bronchial dilators and constrictors. Tobacco is a bronchial constrictor whereas Cannabis is a bronchial dilator; this means smoking tobacco will tighten and close the pathways in your lungs where Cannabis will open them up. Nicotine constricts terminal bronchioles in lungs, which keeps tars trapped in the tissue. Its there these chemicals do their worst. THC and the chemicals associated with MJ are dilators. This allows the particulate matter (black mucus) etc., to be expelled, thereby eliminating their effects "on contact" with lung tissue. Anytime you smoke you’re going to have a presence of mucus.
For me MJ has a slight allergic reaction just like my everyday allergies. However it is this very bodily reaction that protects us from long term contacts with allergens, by allowing "matter" to be trapped and expelled. So, the "Black Phlegm" is a good thing.
(1) THC is a bronchial dilator, which means it works like a cough drop by opening up your lungs and therefore aiding in the clearance of smoke and dirt. Nicotine has the exact opposite effect. (5) Unlike the chemicals in marijuana, nicotine has a paralyzing effect on the tiny hairs along the body's air passages. These hairs normally work to keep foreign matter out of the lungs. This means that carcinogenic tar from cigarette smoke is relatively much harder to purge from your lungs than is that from marijuana.
 Heath, R.G., A.T. Fitzjarrell, C.J. Fontana, and R.E. Garey. "Cannabis sativa: Effects on brain function and ultrastructure in Rhesus monkeys," Biological Psychiatry. 15:657-690, 1980.