Mucus protects our body by trapping bacteria and particles. The color and texture of your mucus can give clues about your overall health. If your mucus is gray or black You likely inhaled dark-colored particles, like smoke from a fire or heavy exhaust. Regular smokers can also blow out darker mucus because of the tar or other toxic byproducts they inhale, says Alfred M.C. Iloreta, Jr., MD, an otolaryngologist at The Mount Sinai Hospital in New York City. If you’re not a smoker, black mucus could mean a serious fungal infection, especially in people with compromised immune systems, according to Cleveland Clinic. Wondering why we even have mucus in the first place?