Unfortunately I've just come back from work to find my betta dead. I'd been battling with some fin rot - of which was just under control and the tail was growing back. This morning (7.30am) he displayed this graphite patchy tail, didn't eat and wasn't swimming. By the evening (6pm) he'd passed. This is the second betta I've lost to this disease now, I'm hoping that some answers start coming through!
Here's some pictures of the disease, though these were taking (perhaps hours) after death.