This is probably a sign of the lungs "cleaning" themselves of the pollutants in cigarette smoke that collected there during the time you were smoking. It may continue for weeks. Usually it begins to taper-off in a month or two.
You may want to look at our giving up smoking information at http://www.nationaljewish.org/disease-info/wellness/smoking/index.aspx for ways to help you quit smoking. Also check with your doctor for other quit smoking resources in your area.
I am curious to find the answer also. I smoked heavy for many years and have quit about 8 months ago when I saw black stuff in my phlegm when I brush my teach in the morning. I still find some black stuff although it's not as bad.
After smoking for 20 or so years and having given up 8 months ago i never experienced having black mucus, now it could just be your lungs having a clear out, but if it dosen't settle down i would go to the dr's.
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