Could Coughing Up Thick Mucus With Black Specks Weekly Something Serious?

Hi I would cough up black specks about once a week it s usually thick and gooey with black dots. I been to multiple hospitals I had multiple test done the doctors keep telling me I m fine but I don t believe it I had a x-ray a CT scan a sputum test and they keep telling me it s allergies. Im worried it could be melanoma in the esophagus, Lungs or mucas membranes the doctors isn t taking me seriously saying I m over reacting I used to smoke weed a cigarettes I hope it s just tar but I don t know I lost hope I m terrified actually planning my funeral please help I m so lost and scared..