You need to show the tongue to a dentist specialist for without seeing it’s difficult to tell why it’s black . It might be because it’s natural or you might be having a fungal infection on tongue.

Hi Burning tongue is due to so many causes Primarily it may due to nerve damage Other causes allergies to specific foods or dental products anxiety or depression diabetes dry mouth gastroesophageal reflux disease geographic tongue an underactive thyroid, called hypothyroidism medications, such as high blood pressure medicines mouth infections nutritional deficiencies biting the tip of the tongue frequently consuming too many acidic foods or drinks grinding the teeth on a regular basis overbrushing the tongue overusing mouthwash or abrasive oral hygiene products wearing ill-fitting dentures