I just got a gun back from the gunsmith that I had reblued, and he didn't do the trigger guard. I know you're not supposed to do the trigger, but is there anything wrong with doing the trigger guard?
What firearm was this? On a shotgun or rifle the trigger groups are generally separate from the receiver and cannot be blued because they are aluminum or some other pot metal.
Take a magnet to the trigger guard to see if it's steel or aluminum. If it's aluminum, it's an unbluable part. There are ways to refinish aluminum, you may want to discuss options with your smith...
Then I can't imagine why it was excluded from the rebluing package, being a significantly visible component...I would ask, it was likely a bona fide oversight.