Duke Blueblood's (which, according to the MLP staff, was meant to be his title) marriage to Rarity is primarily based in headcanon territory, though it's a rather convincing one that I shall share to explain myself.During the episode "Best Night Ever", we see that Blueblood is nothing but an arrogant, snobbish prick. He treats Rarity poorly, ruins her dress, and insults Applejack's food, among other things. It makes us wonder how Celestia and Luna could tolerate such an unruly stallion, and makes us wonder further how Rarity could stand him in the episode "Sweet and Elite"!Well, what if I told you that the Blueblood we met at the Gala wasn't the true Blueblood?Where's the proof? Well, as said, this is all merely a headcanon, but allow me to explain.^ look here at this screenshot from "The Equestria Games". What do you see?-Slightly duller color (though most of it attributes to the shading)-3 crowns for a cutiemark. Fitting for a noble, wouldn't you say?^ now look at this image from "Best Night Ever".-Brighter color-An irregular compass star. Sorta odd for a noble of his caliber to have that as a cutiemark, eh? Not to mention that an irregular compass star is usually a sign of fooling others and throwing them off.Well, someone of the Duke's/Prince's caliber would be considered a celebrity right? ......Celebrities have impersonators.In my headcanon, the Blueblood at the Gala was an impersonator. Someone who managed to worm his way into the Gala in the Duke's place (the real Blueblood probably either late or unable to attend). This impersonator turned out to be rude and snobbish, while the true Blueblood was kind and a gentlecolt, if not just a bit easily stressed.One other point to mention:-Rarity's daydream had Celestia introduce Blueblood to her. However at the Gala, "Blueblood" was staying away from Celestia, causing Rarity to have to go find him. Fitting, since Celestia would probably be able to tell he wasn't her nephew. The timing of it also played a part, as Rarity found him before she asked Celestia. Had she asked, Celestia might've told her that he was away or arriving late.As such, I figure during "Sweet and Elite", Rarity might've come to realize what had happened after getting the chance to meet the REAL Blueblood.(Not to mention we never see his cutiemark in said episode:^ it's due to this headcanon that my lovely bf gave me as to why i ship them X3