The commonest cause of a very thick mucus which is black in colour along with soreness in the throat and cold and cough is because of regular smoking. If you are a regular smoker, you take in a lot of carbon with chemicals, this gets deposited in the lung and in the process of healing itself the body tries to remove these carbon particles and the chemicals. So it starts pushing it out of the lungs. So it comes to the throat and it starts irritating the throat leading to inflammation, chemical induced inflammation causing soreness not as bad as what we would have during infection, but not as bad as tonsillitis or pharyngitis, but it makes you constantly clear the throat. Constant clearing will lead to this thick mucus which is back in color because of the presence of carbon particles in it. rare cause could be some bleeding inside the respiratory tract maybe because of a past infection or because of a tuberculosis or some other lung infection, there has been some other bleed inside the lungs which got stored inside in the form of blackish colour or at some point if you are coughing, it can get dislodged and come in contact with the mucus and give a black colour. If there is a constant black color excess in quantity for a long time, it is better to consult a doctor and get the problem sorted out.
