This is most unusual. The most likely cause is the
inhalationInhalation anthrax of an irritant, which has black particles. It would be wise to make a careful review of all recent occupational and home exposures. Black mold has been described as a cause of infection and lung damage in infants, but I could find no reports of it causing nasal symptoms in adults.
My suggestion would be to have this black mucous examined microscopically and cultured for fungi.