I found this from
http://www.jewelrygenius.com/tarnish.html:
TARNISHING OF STERLING SILVER JEWELRY
Sterling silver is copper-silver alloy and is somewhat softer than most gold alloys. Therefore, it is more likely to be abraded to black dust by relative movement between the jewelry and the skin or clothing.
Sterling silver jewelry has a pronounced tendency to tarnish. The tarnish is always due to the contact with sulphur compounds. The main cause of tarnishing of silver is a trace of hydrogen sulfide gas in the air. This gas is particularly present in smoke from burnt raw fuel, such as coal or oil; in some strong smelling foodstuffs, such as eggs and onions; in some fish or shellfish; and in polluted air.
The tarnish may turn the jewelry black and may also be rubbed off on the skin or clothing. A few people with especially moist skin may find that the sterling silver stains their skin green. This is due to the copper component of the silver alloy. When this occurs you should polish your jewelry frequently and use an absorbent powder on those moist skin areas which come into contact with your jewelry.
OTHER PROBLEMS AND SOLUTIONS
Soap, detergent, wax, polish, and cosmetic creams may build up under a ring, causing a type of dermatitis. A burning sensation and redness of the skin results. To prevent this, you should remove all rings before using soap or detergent, and clean the rings frequently.
The wearing of silver jewelry when manipulating photographic chemicals or solutions may cause staining of the skin. When silver is attacked by a chemical solution such as nitric acid, the solution of silver salt formed is reduced back to silver by the skin. This silver is in a finely divided form and causes an intensely black stain. Silver jewelry should be removed before working with photographic chemicals or solutions.
Medications in the wearer's system, especially certain antibiotics, can also cause jewelry to blacken or discolor. In in some rare cases, the wearer's basic body chemistry reacts to the alloys in both gold and silver and cause tarnishing. A solution to both of these situations may often be had by wearing a higher karat of jewelry: 18K or 22K instead of 14K.
This information is not offered as an excuse, but to clarify the reasons, provide the facts, and offer solutions for those who may encounter the problems described.
Also from
http://saltlakesilver.com/info.html#black:
Customers may think that faulty manufacturing might be the problem when a ring "turns," blackening or discoloring the skin and clothing, or the jewelry itself. However, that is not the case. The most common reason is metallic abrasion, caused by makeup on skin or clothing. Cosmetics often contain compounds harder than the jewelry itself, which wear or rub off very tiny particles. Very finely divided metal always appears black rather than metallic, so it looks like a jet-black dust. When this dust comes into contact with absorbent surfaces such as skin or clothing, it sticks, forming a black smudge. To prevent this, try switching cosmetics. If this is not possible, remove rings and other jewelry while applying them, and clean skin areas in contact with jewelry with soap and water. Another cause is actual corrosion of the metals. Silver does not corrode, but its primary alloys will do so, forming very dark chemical compounds under moist or wet conditions. When you perspire, fats and fatty acids released can cause corrosion of silver jewelry, especially when exposed to warmth and air. This problem can be worse in seacoast and semitropical areas, where chlorides combine with perspiration to form a corrosive element that discolors skin. Smog fumes gradually attack jewelry and are evident as a tarnish that rubs off on the skin. Remove jewelry often and use an absorbent powder, free of abrasives, on skin that comes into contact with jewelry. Even the design of jewelry can be an influence. Wide shanks have more surface area to contact abrasives or corrosives. Concave surfaces inside a shank form collection points that trap moisture and contaminants, also causing a type of dermatitis. Remove all rings before using soaps, cleaning compounds or detergents, and clean the rings with a polishing cloth frequently (we offer one for a very small price). As well as solving the problem, you’ll be amazed at how much better the rings look!