During the duel against the Shaman, about four turns in, the Shaman, the judge, and I disappear, and it says "You find yourself surrounded by darkness," but the screen doesn't go dark.

It's a bug, but I think casting the appropriate spell should still work.

The shaman casts a shroud of darkness over the area at that point in the duel and the screen is supposed to fade to black as a consequence. You should be able to continue as long as you cast the appropriate counterspell, which would normally cause the screen to fade back in.