(Of course, who really has a dinner jacket these days? I can't imagine they mean the traditional definition...)QUOTE]
Wow! I must be a real anachronism! I mix up my dinner jacket and tux jacket for the two formal nights, with a cummerbund on the former and a vest with the latter.
I'm sure they mean the "traditional definition," as that's formal; a blazer is actually informal attire, as opposed to a sports coat, which is casual (the difference being the cut and pattern of one versus the other--blazers never have a pattern, and have metal as opposed to bone, wood, leather or plastic buttons).
That said, a blazer would be a better fit in the dining room than a sports coat, but is still not formal--not that anyone will faint or cry "foul" no matter what you wear!