Rhodium is one of the most noble metals, meaning that it does not
oxidize easily, like gold and platinum. You will have no success
trying to darken it.
You can, however, remove the rhodium finish by dissolving it in a
strong sulfuric acid solution, heated of course. Then, after thorough
cleaning, remove the nickel underplating in hot hydrochloric acid. If
the piece is silver, then it can be oxidized.
Chris van Laer