A piece of blackened or oxidized silver is sterling silver, but its surface has been intentionally darkened 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.
Silversmiths use a chemical compound such as liver of sulfur, 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 a 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.