Black discoloration of the tongue is due to poor oral hygiene , any amalgam restoration in the mouth , vitamin and iron deficiency, aneamia , medications, dry mouth. You can consult a dentist and know the cause and get it treated.

The black discoloration on your tongue from this picture looks quite normal. You may consult a specialist in Oral Medicine.

Black discolouration on tongue is either caused by some medication, smoking, vitamin deficiency, bad oral hygiene or dry mouth. If you are a smoker then stop it. Prevention is better than cure. But if you are having lumps, swallowing problems or feel swelling on tongue on intervals then please see an oral surgeon, he will treat you with utmost care. Because with just pictorial representation of your tongue and not having any tests done, it is invalid to come to any conclusions. Good luck.