Optics. Because teeth are translucent, they can reflect the light in such a way as to give the appearance of darkness at the junction of the filling. As the filling ages, it can also stain the teeth. A new cavity could also increase the darkness around a filling. You should ask your dentist.
Thin enamel. The enamel along the edges may be thin , so the dark color of the filling shows through. Also, as the fillings age, they discolor and this shows through.
Staining. This is a function of how the dentist has to drill out the decay and the age of the filling. There could also be some new decay under the fillings. There are newer restorative materials that are more esthetic, so if this is of concern, you should discuss this with your dentist.