That black mark looked awfully familiar, so I hauled out my ARC Firefly/4F - a unit that has seen a lot of use, mostly about fifteen years ago. It has a black band in the same location. I suspect that it is perfectly normal. My Firefly has had the black band for as long as I can remember, and it has never, ever failed me.
Now, if one of our informed tech types (Ayres, are you there?) could just give us the real skinny on this phenomenon, we could all rest easy.