The vet sedated him and looked and felt around in his mouth. As usual, Noble wasn't the most cooperative patient - he's very nervous around vets. Although she said he could do with a bit of a float by our dentist at his next visit, there weren't even any really sharp or protruding points. He does have one lower molar on the left side way towards the back that is a slight bit loose, but it wasn't broken or abscessed and she said it shouldn't have caused the inability to eat that we observed. He does have old teeth - not surprising at 29 - with some places and pockets where food could get caught or trapped, but his teeth are really in pretty good shape.
Who knows? Perhaps he banged something playing "mouth tag" with Scout, or had some food stuck in a tooth pocket. We did a blood draw so she could run a basic chemistry panel, including cell counts and liver and kidney function. Our dentist will make a regular visit in several months and will take another look then. As long as he continues to improve and eat his mash and enough hay, and doesn't get worse, I'm not going to worry too much. Also, he's been on his thyroid medicine for about two weeks, and it could be that he's also feeling somewhat better because of that.
The vet also rechecked Blackjack's eye, which is looking very good. We'll continue his eye antibiotics through the weekend just as a precaution.