Black color patch in the mouth is seen in:

Vincent's Angina: (Ulcerative Gingivitis/Trench mouth) Was common during first world war (due to lack of oral hygiene) and is less common now. Caused by fusiform bacillus and spirochetes: Borrelia vincentii. It manifests as necrotizing gingivostomatitis with oropharyngeal ulcerations and dark gray membrane. Membrane generally present on one tonsil but may involve the gum soft, and hard palate. It appears as grayish black slough which bleeds when it is removed. Ulcers are visible on tonsil after removal of membrane. Membrane reforms after removal. Characteristic smell in breath (halitosis), so also called as Trench mouth. Treatment Systemic antibiotics: Penicillin, Erythromycin, Metronidazole. Warm sodium bicarbonate gargles. Barrier nursing of the patient as disease is infectious.