This has happened to me several times now, on different characters. What happens is that after pelting the blue dragon with arrows until it moves to its next location, Biorr is hostile when I get close to him. He still talks like usual, and you can use 'talk' (press X) when fighting him up close after killing the dragon.
- The first time was on a Pure Black run through world 1. I had previously been to 1-4, but didn't bring enough arrows to kill the dragon, so I left Biorr halfway through the fight and went back to the Nexus. Then I turned the world Pure Black, and ran through. I took out BP Miralda, Red Dragon, Primeval and proceeded into 1-4 to kill the dragon. When I arrived at the stairs in 1-4, Biorr asked if I needed any help, then killed me (was in hypermode without ss so he one-shotted me). I respawned at Penetrator AS and ran back to Biorr in soul-form, and he was back to normal.
- Today the same happened at the end of a pure black run. When the dragon moved I ran after, but Biorr kept attacking me while talking to me at the same time. I just ran past him and killed the dragon from the door, and then killed BP Ostrava. When I got back to Biorr he was still mad, but I turtled around him while exhausting his conversation and he finally sat down and fell asleep. lol
- Retried this on another character/playthrough, but this time I started at Penetrator AS during PBWT. After the dragon moved to the second location Biorr was aggro, but there were no option to talk to him (probably have to kill the dragon first). Tried this a few times in a row, each time I was able to target him when I got in range, and he completely ignores the dragon to chase me instead. When i returned to the Nexus, he was sitting in his usual place, calm as ever.
- Tried to let him kill me then returned in Soul Form, but he was still aggro.
- Turned the world White, then went back, and he was finally ignoring me again.
- Once more through, PBWT, and still aggro when I got in range. After this I took out BP Miralda and the Red Dragon, then proceeded towards Tower Knight. But I managed to die, while dodging arrows at the end of the bridge, when I rolled over a broken cart and over the side of the bridge. Respawned at Phalanx AS, used a SoEE and ran through to kill the Primeval Demon. I continued through 1-4 to kill the Blue Dragon as well, and when I reached Biorr he was ignoring me again. World was still Pure Black.
- One more Pure Black run, and he was aggro in 1-4. But this time I also killed Penetrator on the same run, hypermode bow and arrows. I noticed Biorr was just standing still in his location without helping me. Penetrator still attacked him when he got close, but Biorr didn't retaliate.