It is natural for sterling silver jewellery to tarnish, blacken or discolour over time, particularly if the piece is worn regularly.
Tarnish is caused by several factors which include:
Strokeside Designs advises against:
Wash tarnished item with a mild soap, then place the piece on aluminum foil in the bottom of a pot. Add a baking soda solution of 1/4 cup soda and a few teaspoons of salt into 1 quart (4 cups) boiling water. Cover for a few minutes. Then rinse and let dry thoroughly.
If in doubt, take it to a jeweller! At least once per year (more often as the item ages) take it to your jewelry for a check up. Jewelry is just like a car and needs to be maintained and occasionally rebuilt/reinforced.