While silver naturally has a bright finish, a dark gray/black finish can be equally desirable. I am going to cover a couple methods to give your bright silver jewelry (or copper) a nice dark finish. Keep in mind the patina is just a reaction on the surface of the metal, so it will wear off when rubbed against hard surfaces. However, you can always apply a clear coat to make it last longer.
Dark Patina - Home Method
This method is a simple solution requiring just one hard boiled egg and a small ziplock baggie. It works for both sterling silver and copper.
First you will want to make sure your jewelry has been cleaned of all dirt and oils by washing it with warm soapy water, then dry. ( I like using dish soap.)
Next take a hard boiled egg and peel off the shell. Place the entire egg into the plastic baggie and break it up into small pieces.
Then take your jewelry and place it inside the bag with the egg. You can just place it right on top of the crushed egg. Close the bag. In about 5 minutes you will notice the piece changing. Sometime the colors start with reds, then blues and gradually get darker. In about 10-15 minutes it should have a nice gray tone. You can take it out at any point when you like the finish, or leave it in for hours to see just how dark you can get it.
When it reaches the desired tone just take the jewelry out and rinse it off.
Now the patina is going to wear off as it is rubbed against hard surfaces. The high spots will get the lightest. This may be desired to accentuate a texture, such as the image below. You can even rub the piece with fine steel wool to bring the highlights out faster.
If your hope is to make the entire piece stay dark longer you can add a clear coat. If you are in a pinch clear finger nail polish will work, otherwise you can pick up a clear coat spray/dip from your local hardware store.
Dark Patina - Liver of Sulfur
If you are making a lot of jewelry with a dark patina, liver of sulfur is the way to go. Make sure you wear plastic gloves and have good ventilation. The liver of sulfur comes in a small metal tin and the pieces look like rocks. ( I get mine from RioGrande.com ) You'll want to break the chunks down into smaller pieces (roughly the size of a red pepper flake - haha that is all I can think of ).
As before the first thing you want to do is clean your jewelry with warm soapy water and dry.
Next, fill a small dish with warm water. Add enough water so you can fully submerge your jewelry item.
Add one small piece of liver of sulfur and dissolve. Then immediately add your jewelry. I use rubber coated tongs and make sure to gently rotate the piece every 30 seconds or so.
You'll notice the piece almost instantly start changing to red/brown then blue/purple hues. If I am adding a dark tone to a ring, once it gets the initial dark gray tone I like to take it out and use a soapy brass brush or soft toothbrush to gently rub the patina. It kind of adds a smooth tone to the piece. I then rinse and put it back in the sulfur water until it darkens to the finish I am looking for.
Take the piece out, rinse it in cool water and dry with a blow dryer. I like to add a wax sealer, called Renaissance Wax, which makes the finish look rich and dark.
As before the finish is only a reaction that occurs on the outer surface so it will wear off. You can rub gently with fine steel wool to highlight a texture. If a clear coat is desired to keep the piece dark longer, I recommend ProtectaClear. You can brush it on or dip the piece for a pretty durable coating.