Yep, silver comes out either black or silver. Weird. In this case, since the thimble has a patina, I would expect it to be black and not the color it is if it were silver. Most time, silver thimbles have some sort of decoration. If you had the money to buy a silver thimble, you could afford a few more dollars for decoration. A way to charge more and also show people you could afford a fancy silver thimble.
Silver is the one thing that the experts on Antiques Road Show say it is OK to clean. I guess since the "patina" on silver - if it has one - is black and not that attractive green color.
Daryl