Stain From A Rubber Band

An acetone rinse may have some beneficial effect.
If not, you're pretty much going to have to leave it the way it is unless you want to destroy the original surfaces. Rubber bands contain sulfur, one of the strongest coin staining substances. No cure for it once darkened that does not involve loss of natural toning.

The "stain" comes from sulfur in the rubber. It is sulfur in the air and on people's fingers that causes silver to naturally tone or tarnish; so your "stain" is chemically identical to the rest of the "natural" toning on the coin, which you would presumably prefer not to remove.

So the sad news is, you can't remove it without removing the rest of the toning and creating a "cleaned coin". It is up to you whether you prefer to own a coin that has "rubber band stripes" or a coin that is "cleaned", but this coin no longer has the option of being neither. If they were my coins shipped like that and I knew the coins didn't have the stripes when I bought them, I'd be ropable; their shipping methodology has ruined your coins.

Personally, I'd prefer leaving it as-is. I hate the cleaned look worse than I hate black stripes.

Sulphur is commonly used in the vulcanisation of rubber.