You may or may not have noticed it, but I really like to write about Blueblood. For all the lack of anything he has in continuity, I really gravitate to him as a character. And because of this, I like to give him stuff. Like a sister, a dark destiny, and a kingdom of his own. To elaborate on each:
The Sister: Cadance. She’s been put down as Celestia’s niece. Blueblood is her nephew. The most logical course of action was have his parents adopt her, since she couldn’t for whatever reason (age limit?). Ad since Cadance alicorning/adoption happened when she was (about) CMC age, it’s feasible that they have known each other for a very long time. And I think they love each other. Which to me makes poetic sense.
Blueblood, a character, a creature designed to be hated, would of course be loved by the princess who had the had the most love to give.
Will explain the others in another post.