Hi! You might want to consider using Rio Grande's "Black Max" oxdizer. I have used it on brass, bronze, & copper and it yields an intense, opaque black finish. I haven't used it on silver, as I prefer liver of sulfur, but I think it would work. Also, if you're using liver of sulfur, you might want to heat up your piece, until it's warm, & dip it in your solution to accelerate the coloring.
As for finishing, rinse the piece in soapy water with diluted baking soda to remove any liver of sulfur residue that may continue to develop. With Black Max, I've only covered it with a layer of wax, after it had dried, so it wouldn't rub off.
I hope this is of help to you.
Peggy