Silver will tarnish over time. This does not mean that the keys or silver are damaged. What you see is just the oxidation of silver, a natural chemical reaction. The sulfur interacted with the silver and created a black/brownish-looking patina oxide. To remove it, you have 2 options:
1. You can remove it physically by rubbing it off with some kind of silver cloth or tincture, but this will always remove the silver from the keys and the plating will eventually be gone.
2.A better solution is the chemical sulfur extraction. The silver will be unharmed but it should be done in combination with an overhaul because the keys need to be prepared for it.
1. The keys need to be removed from the instrument
2. All pads and bumpers need to be removed unless they are LP silent pads (These can be cleaned with ultrasonic)
3. The keys will be dipped into the solver removing solvent and
4. Need to be perfectly cleaned, best with ultrasonic, from all remains of this solvent which is very aggressive towards steel and would easily damage the screws and rod.
5. There is a silver-protecting coating available, which we can add to the process. It will prolong the time the silver will stay shiny, but it will not last forever.
Our advice is either to let it tarnish or if you would like to remove the oxide, do it very carefully and seldom.
Silver tarnish prevention
Nevertheless, you can try to prevent it from happening by putting a so-called "silver protection strip"
inside your case.
In addition, you should always wipe off your fingerprints after you play the instrument and put it back into the case
Most mouthpieces are made from a hard rubber which includes a high percentage of sulfur. If you notice, that your keys, which lay close to your mouthpiece compartment tarnish, you can be sure, that they are the reason.
The sulfur will evaporate over time but especially new mouthpieces should be kept away from the instrument. Put them either in a separate mouthpiece case or into a closed plastic bag, while inside the case!