one of my

It would be useful to see a photo of the tooth but in the absence of that this my reply makes some assumptions from what you say
If a "silver" changes colour it is usually due to corrosion of the metals it contains and the dark margin is likely to be further evidence of this corrosion
Sometimes old "silver" fillings tarnish (as they have a high silver content which like tableware reacts with certain foodsuffs)
As others have suggested it is worth checking this
Teeth that are changing colour are best investigated as this corrosion may weaken the tooth or filling

It is best that you arrange an appointment with your dentist,who will be able to provide an answer and treatment if necessary,supported by examination and possible X-ray of your tooth.

Any discolouration like this should be checked by a dentist and see a hygienist for a good clean , when you see what it looks like after and depending on the outcome of your dental examination, you may find it is fine, If it needs to be redone consider a holistic approach will remove your amalgam safely