TMI!!!!! black tongue

i noticed green patch on my 7 months old baby but since yesterday it has turned black.... ive tried to scrap it off but it isnt coming off.... we have seen two gps but both are saying its fine its just gross but it will go back to normal on its own.... my gutt is saying something is wrong. has anyone of u seen the same..... UPDATE - so the second day when my baby was sleeping i opened his mouth and saw really tiny and thin hair on his tongue.... As gross as it could be i rushed to the doctor and he gave him Nystain but again he said its normal and there is nothing to worry about,,, so after a day of applying nystain nothing happened so i mixed an ounce of distilled water with 2 drops of grape fruit seed extract and i started cleaning his mouth with this after he eats anything and also used nystain.... so now yesterday when i cleaned his mouth everytime some black stuff was coming out alongwith few hair... GROSS!!!! it hasnt gone but has reduced a bit I am hoping to see some visible difference by day end if not I will again go to doctor tomorrow...