When the fillings get old or when they are not merged properly with the tooth structure then the gap develops between the tooth and filling causing the gap and here micro leakage occurs and the secondary caries develops. For this you have to go to dentist and get clinical exam done. sometimes this black color is due to roughness around the filling and as you drink lot of coffee it gets stained. So after clinical examination you dentist will find out and if there are secondary caries or micro leakage then you have to get the filling changed and if it is due to staining then polishing of filling will serve the purpose. for more info on fillings you can read this article... http://www.identalhub.com/article_advantages-and-disadvantages-of-composites-42.aspx