The act of ruining a perfectly good TV show with a terrible "twist" during the series finale; referring to the eyeroll-inducing "twist" during the series finale of How I Met Your Mother, where the entire story of how Ted met his children's mother turned out to be an unconscious attempt to ask his children if he can date his ex-girlfriend Robin again. Ted then shows up at Robin's house with the blue horn from their first date/the first episode.
"Man, I used to love that show, but the series finale was such a blue french horn."
"I can't believe how they blue french horned the TV show like that!"
"Remember when that entire season of Roseanne was just a hallucination? What a blue french horn!"
"They thought they were bringing the show back full circle, but really they just blue french horned the entire thing."
To bluehorn something. To completely screw something up in the last second, even though it was pretty much a guaranteed success already.
Has its origin from the series finale of HIMYM, that was described by many people as one of the worst (if not the worst) in recent TV history because of its unnecessary and mostly unwanted plot twisting, which a blue french horn is symbolic for in the shows legacy. Hence to bluehorn something.