There is a black line alongthe edge of my composite filling. is this indicative of a problem, or is there a chance it could be benign?

Stain: If the line is along the edge of you composite filling it is probably just stained from foods or coffee or dark sodas. Could also be decay...but to be safe...GO back to your dentist and have it looked at...it might just need a smoothing out or cleaning up the edges. If it is an old filling...might need replacing.

See your dentist : It sounds like material from your saliva and food has worked its way under the margins of your filling. The dentist can repair or replace it.

Stain or decay: See a dentist and have the filling checked. There could be some stain that may be polished out, or it could be decay and replacement would be needed. The only way to tell is with a dental exam.