What Causes Black Discoloration On The Tongue?

my father is 72 years old. He is suffering from mouth thrash. He has been prescribed vizylac, candid mouth gel and flucos to take. His tongue has improved after a long treatment with these drugs but i have found there is light blackish color present in the middle of the tongue. Once an ENT specialist had suspected my father suffering from cancer but other doctor is now diagnosing and told me to wait for having any conclusion at an early stage. Please help to know if there could be any danger to my father's health.