Sterling silver is called '.925 sterling silver' because it contains 92.5% fine silver, the remaining 7.5% is copper, which makes it stronger/harder and also has the unfortunate side-effect of causing it to oxidize when exposed to moisture and air.
When sterling silver oxidizes (due to exposure to moisture, air, and chemicals), it starts to turn black. Some folks have highly acidic skin which can oxidize sterling silver faster. This is not an allergy. Please be aware that this is not a defect of the product. (Miss Annie, our founder, has this issue with anything sterling silver, she just keeps her pieces clean and polished.)
Often it just needs a buff with a jewelry polishing cloth to look as good as new. We are happy to polish, buff and refinish sterling silver jewelry for a fee. Many jewelry stores can do this, as well.