Silver Black, which is mixture of hydrochloric acid and tellurium, is a product that has a similar affect as liver of sulfur in that it oxidizes metal. This means that it's akin to tarnishing, only you are doing it on purpose and controlling which parts turn black and which do not. The reason why this affect is used sometimes by jewelers is to help show off details in a finished piece of jewelry. By darkening some areas of the metal, the detail is much easier to see. It is also a way to create an antique or patina affect, giving jewelry a look of being an estate piece rather than something brand new. Many jewelry suppliers such as www.riogrand.com
sell this an other oxidizing products.
To use Silver Black, cover your work area with an old towel, have some paper towels nearby, and set out a small bowel of water and a sponge for application purposes.
Make sure you are working in a well-ventilated area. This is a toxic chemical, so you want to make sure your children or animals can't be exposed and that you aren't breathing in the fumes.