Looks like something caused it to lose its skin pigment, if it happened quickly then maybe something stung him. I've never seen that on a trigger. Hopefully it goes back to normal.
Not unlikely ... we often see similar marks (though darker, on a light background, called "hypermelanization") pretty commonly on clownfish that attempt to guest in, for example, frogspawns or hammers.
Ok. I wrote to Bob Fenner (wetwebmedia.com and author of Triggerfishes for Marine Aquariums) and he thought it was "burn" by bristleworms. I have a lot of those and this blue throat is new to the tank (2 - 3 days). I guess it has learned a lesson. This morning the burn seems to fade away.
Thank you guys.