The following techniques are used for blackening Sterling Silver. When blackening pieces made out of sterling silver (925 pure silver), since not all sterling contains the same alloys in their composition, you will notice that reactions to dipping solutions may not be the same for all the pieces that you are trying to oxidize. The pieces that you are blackening must be fully finished, all scratches removed ,and fully polished or textured to the final look that you wish to maintain when finished.
Liver of Sulphur
One of the most common techniques is the use of Liver of Sulphur, a commercially available product that comes in the form of small solid rocks.
Dissolve a small rock, about the size of an almond, in warm water and you will end up with a yellowish solution that is very effective for the oxidation and blackening of Sterling Silver. The smell from this solution is not going to be pleasant (it smells like rotten eggs), make sure that you do not inhale directly above it. The next step is to dip the object to be blackened in and out of the warm solution in short intervals of about 10 seconds each, until you get the desired depth of color. This finish is on the surface and it only holds fairly strong in recessed areas of the object. This solution also works well on copper.
Regular chlorine also works as an oxidizer for silver; it takes a little longer to act than other solutions and goes through a slow coloring grayscale as it is applied to the piece. Again, dip the piece in intervals of 10 seconds each and observe the change of color until you get your desired shade of gray or black. The black coloring can easily be removed by lightly polishing the piece. This method does not seem to work well with Argentium silver.