Silver in a purest form is a reactive metal and so soft in nature that mostly everything will bend if made out of pure silver. Over the period of time metallurgy has evolved a way to counter this issue, they add copper.
Adding Copper to silver not only makes it hard but also make its wearable and less reactive. But copper is also reactive, but less reactive than silver.
Even kept in open air at room temperature, over the period of time, pure silver (sterling silver) or silver mixed with copper start appearing black or yellow as it slowly react with moisture in the air, oxygen etc.
In some way its a good sign, which means that metal in your hand is pure. So to keep your silver in original condition, it is important to keep it away from air.
That’s why most of the mint made coins come in a airtight pack. So if you have silver bullion or any other coin, store them in an airtight capsule. Don’t leave your coin in open for a while.