A silver horn is going to tarnish at some point. The more it is handled and the more it left exposed, the faster it will tarnish. My own horns have shown tarnish that is black-ish and also that is yellow, depending on the horn and conditions. You could always buy a polishing cloth from Ash, one that is treated with polish on one side and just plain on the other side. Then use it on one of the silver patches and see what happens. I suspect it will be satisfying. There are many threads on the forum about polish, but basically you want to go over the horn with a good silver polish that includes a tarnish inhibitor. I always used Hagerty polish with great results:
https://www.amazon.com/Hagerty-10120.../dp/B0000CFMQO
That said, because Wessex is always improving things, it is worthwhile to see how old that floor model is. They are inexpensive enough that one does not need to feel constrained to buy a used one or an old/new one.
Almost any new horn you get at any price will need a breaking-in period for the valves, no matter how good they feel out of the gate. I always remove the pistons daily, wipe them down, and oil them for a couple weeks at minimum. AND when putting the piston back in, you must be sure the valve spring is seated correctly, so the cylinder should be straight up and down as you do this. If not, the spring could be crooked and scrape the sides as it moves. My own horn would cost close to $10k new and I have some high-priced "Mead Springs" in it to replace the standard springs. But if I don't keep it clean and keep the springs centered, it will make scraping noise.