It's tartar . Get scaling done by nearby dentist .

Hello dear Please be aware See it is nothing but dental calculus which is below your gums.it needs scaling or curettage.for further details you can consult me Thanks

Hello This blackening of the gums is nothing but deposit of calculus that occurs or gets accumulated due to our daily dietary food items that we eat and drink. You need to go for a round of teeth scaling and polishing and this will be removed. Ideally scaling has to be done every six months because otherwise these deposits will keep on accumulating and ultimately lead to severe gum problems like periodontitis, bone loss and sensitivity.

Go for scaling and polishing

It is tartar which is situated below the gums.  Whenever tartar is located below the gums,  it takes greenish or darkish color.  It will be hard and can't be removed by brushing and it has also caused some inflammation in your gums in your case.

From photo it looks like it's probably tartar(plaque when remains on teeth for longer time it turns to tartar). Get it checked by doctor.Scaling and polishing will solve your problem