I dipped a piece of chain which appears to be silver plated into liver of sulphur and did not get any reaction. Could this mean the chain is rhodium plated? The chain is over 20 years old.

 
JoAnn 

 I should have added the fact that I used hot water, not boiling and other pieces did blacken but not this particular piece of chain.

 Rhodium plating is much whiter, brighter looking than raw silver. Rhodium plated silver looks like the color of 14kt white gold. LOS won't touch it. However, a lot of chains are coated with clear anti tarnish lacquer to prevent the chain tarnishing until the lacquer wears off.

You can remove lacquer coating with acetone. Removing rhodium plating is not so easy. If  the chain is rhodium plated I'd suggest replacing it if you really need to blacken the chain.

 Russ, thank you. I will try the acetone trick.

 JoAnn