Silver can rust or tarnish over time because of exposure to moisture or sulfur in the air. Tarnish and rust can cause silver to darken and look like it is black.
To clean silver that has rusted or become tarnished, line a pan with a piece of aluminum foil, put the silver on the foil, boil some water and add to the water either baking soda to clean tarnish or vinegar to remove rust. Then, pour the water on top of the silver, completely submerging it. Depending on how tarnished or rusted the silver is, results can be immediate or may take several minutes.