I have found this sometimes when part of the silver seems to have a tendency to darken. Maybe it's because of an impurity in the silver in that place, or maybe it's due to something in your case that is touching the horn there. Check out the case first (including a gig bag if you use one). Any contact with leather can cause this, for example.
If that is not the trouble, then I would try the silver dip:
Hagerty Flatware 17-1/2oz. Silver Dip
You can let this sit on the area affected for a few minutes (read the directions and see what they suggest for stubborn spots). It is probably more gentle than rubbing with standard polish. Once it is clean, use the regular polish to protect it in the future.
Do this area carefully, and in progressive steps with the dip. If the cause is an impurity in the silver or an impurity in the brass underneath, it's possible the plating will be more fragile. Get the black off before you use the normal polish.