Question

silver articles turn black when kept in the open for a few days due to the formation of ________?

Answer 1

A piece of blackened or oxidized silver is true sterling silver, but it’s surface has been intentionally darkened through by introducing it to a chemical process. This patina, a surface color that occurs when the silver is introduced to sulfides, is a sped up version of the natural tarnishing process. A layer of silver sulfide forms on the exterior of the metal giving it a blackened look.

A quick aside: The term oxidizing is a misnomer since the process is caused by the introduction of sulfides and not oxygen. But nevertheless, the industry terminology has stuck although it’s not technically correct.
We jewelers use a chemical compound such as Liver of Sulphur, a potassium sulfide, to create the blackened surface on silver jewelry. The range of colors that can be achieved from this process is extensive. At full strength the color will appear a matte gunmetal black. But with controlled application, a whole rainbow of colors can be achieved including blues, purples, yellows and reds.
Like other patinas, oxidized silver is just a surface treatment and does not change the internal color or properties of the metal.
 

Answer 2

It turns black due to oxidation.