Answer:
It could be due to some really serious diseases or disorders, so you should go to the doctor to find out what is causing yours. Old blood turns from red to black-looking, so if you have had a really bad infection like bronchitis, it could be that you have had some minor bleeding from the infection in your lungs or bronchitis. Blood from your nose and sinuses can also mix with your nasal mucus, run down the back of your throat with post nasal drip and look black when you cough it up. There are many much more serious problems that can cause it, so it should not be ignored as a warning sign, have the doctor examine you.
Black-looking phlegm you cough up can be residue from smoking that is stuck in your respiratory tract. It includes the tar from the burning process and can be caused by both marijuana and cigarettes. Smoking both is the ultimate bad choice. I've been coughing up black phlegm for about a month or two now and am quitting cigarettes and marijuana for good.
Face it, burning and inhaling smoke from anything can't be good for you, so get the will power to stop. The doctor can help you find help to stop. The longer you wait, the worse it will get. Waiting until later will cause you to become more addicted and have much more difficulty stopping.
Smoking isn't cool anymore...
It is either old blood, dead tissue, a substance from out of your body or a black mold infection.
Black mold infection is a dangerous disease that should be treated immediately.
If you want to inhale substances without getting tar and other filth in your lungs, use a vaporizer or e-sigarette instead.