Since you are c/o the filling done in front tooth,the dark line has to be removed after consulting with a dentist
The filling has to be either replaced with a tooth coloured filling or veneering or crown has to be placed.
Depending on the depth of cavity,treatment can be planned.
Please do visit a dentist for further reference.
Maintain oral hygiene
Brush your teeth using soft bristled toothbrush.
mouthrinses to gargle.
Hope this helps.