Okay, investigations of scratch-gate continue.
If we use the Series 10 Apple Watch, the model which used polished titanium, it seems the black finish did struggle for some people:
"Mine scratched the same day I bought it. Exchanged for natural Ti instead. Anything can scratch. The issue is that with dark colours, it becomes more apparent."
I think I'm going to go with one of the lighter models on this basis. As sexy and stealthy as the black looks, as someone who doesn't and won't ever use a case, I'd be annoyed if the black rails had visible scratches.