So, I had a moment of panic with Tal last night. He had bloody tears from each eye, and the eyes themselves were entirely black. It didn't seem to affect his sight (I tested him with the Summoner Wand of the Red Dot Taunt. It worked as normal, he instantly got +4 attack, stealth and charge. And yes, I've been playing FAR too much Hearthstone recently.)
So I planned to take him to the vet this morning, but he's totally back to normal. So I'm going with either he went for the wrong spider and got spat in the face with something nasty but temporary, or he's a vampire. Probably the latter.