I feel pretty confident on the ammonia tests given the pond is well established, we're feeding sparingly and the other fish seem happy. I tested ammonia many times (as a sanity check) with it always coming out the same as tap water (quite close to yellow and we use prime, my eyes aren't great with colour but wife agrees).
Looking at the mark more, I think you might be onto something with the injury theory. The very strong/localized nature of it leans me that way over an ammonia burn (which usually looks a bit more scattered, at least in my experience). Perhaps he injured himself and picked up a secondary infection.
Injury-wise I didn't spot it before due to the top down view of the pond. The previous owner also used blue dye so it isn't easy to visually inspect. I'm beginning to notice that there are ornaments at the bottom though that I'll check for potential injury risks. I'm also going to slowly eliminate the blue dye so I can watch everyone more too.
Blackspot is another great point. We don't have any snails but being outside we can't rule out a parasite. Everything I've read though says it is pretty rare and isn't super deadly (unless I'm wrong). I guess I'll monitor the other fish closely and dose with Praziquantel if concerning symptoms emerge.... we're also heading into winter so hopefully any flukes will be shutting down / dying off.