Silver blackening (oxidation) occurs at random, but usually in response to a chemical reaction (usually in the summer, the combination of sweat, lotion, perfume, soap) and also depending on a possible disease (inflammation of the body). In any case, this is not a defect, but a normal characteristic of this precious metal. A thin oxidated layer can be routinely cleaned with a polishing cloth for jewelry, however If the coating is thick, we recommend professional cleaning at the goldsmith.
Allergies to precious metals is quite common in today's society. One option how not to give up wearing silver is rhodium coating. The rhodium, which it is also a precious metal, has not been found alergenic yet. Rhodium silver does not suffer of blackening. To clean rhodium coated jewellery, use only a soft cloth dipped in lukewarm water with a drop of detergent, rinse and dry immediately!