Why silver tarnishes black
Silver has the characteristic to tarnish black in course of time.  
But why ?  
The fine-silver that is the main constituent of all silver-alloys reacts with the hydrogen sulphide of the air and  the black silver suphide is created in this process, and this tarnishes silver.  
The black layer can be removed by using certain silver-cleaning detergents, thats are available in many jewellery-shops.