Back from the vet and she said that indeed he is rather underweight and needs to build up weight (I, personlly, would like to see him put on about 2 pounds, but even just 1 pound would put him more in the right weight category). She said there are several possibilities (including the dreaded FIP, which we lost his brother Sage to over a year ago) BUT that of all the possibilities, the one she thinks is BY FAR the most likely is exactly what I was thinking -- the change in diet. Blaze has always been a picky eater (not so with Skye or Sienna) so he was barely keeping his weight when on the kitten food, which has more fat to it. Looks like adult food just isn't gonna cut it for him.
So, we've got some ideas to try to get some more fatty-stuff down him. The one I'm going to try tonight is making some homemade chicken noodle soup (no onions) with peas and carrots and boneless chicken, then toss in a handful of the dry kitten food we still have so it can add to the "mush" and take up some of the room of noodles while adding nutrients and stuff, and see if Blaze will eat a full serving of that.
In the meantime, I'm on the hunt for other kitty recipes that we could give Blaze.
Switching back to kitten only for everyone isn't an option because of Sienna (who seems to keep gaining, which says poor Skye, I'm going to have to just stop the canned food), and going to structured feeding isn't going to help either I don't think because Blaze is a grazer. He nibbles on the dry food off and on all day, so if we take away his food for the day, he'll probly only get worse. Though I may try that, too...
Anyway, we're to try him on this other diet of increased kitten chow for a month and see if there's any improvement. If not, or if he just gets worse, then we bring him back in for some bloodwork.