Haven't seen this guy in a while, have we? And no other sign of Equestrian nobility or royalty beyond the alicorns, either... maybe he's just CALLED Prince? Or do you have any better answers?

By virtue of his direct descent from the line of King Astron, last independent monarch of the Kingdom of Canterlot. (The family genealogies trace all the way back through Princess Platinum to Queen Majesty herself, but several of those claims are disputed; Astron is invoked because the Royal Sisters guaranteed his and his heir's titles in perpetuity during the Reconsolidation.) His eldest child will also be a (non-Royal) Prince or Princess, while any others will be Lords or Ladies, the latter also being the correct title for his wife.

30. How did Prince Blueblood get his title?
His family bloodline dates back to the old Unicornia royal line. His being Celestia's "nephew" is an historical fiction.

7038846
Basically this. Calling him 'nephew' -- his line has had this title for generations, never 'grand-nephew' or the like -- helps legitimize a secondary princely line while also keeping them in their place. Which is to say, "You're not the Prince of Equestria, you're just related to the ruler."

Someponies begged a foal from Celestia, but unlike the Cakes they were not infertile, they were just social climbers. Celestia agreed on the condition that they both agree to become infertile; they agreed. They got Blueblood, but they bragged about it. That was a big assumption, because Celestia didn't acknowledge Blueblood as having any courtly power, but the parents were already pushing his title. Their social climbing BS, made it so that they spend all their money keeping Blueblood in the castle. Before Twilight came along Celestia was trying to teach him just how to be nice, but he was already caught up in his perceived greatness. Celestia considers the situation a victory because she put an end to a bloodline of selfishness. Blueblood is not infertile, but because he is practically confined to the castle where the ponies who could get to know him are less likely to be wowed by his “position”, there is no chance of his having offspring until he changes his ways drastically. Celestia is also aware that Blueblood's current attitude is a wake up call for social climbers trying to weasel their way into popularity.

Right. This is one I'm fond of.

Prince Blueblood is actually the crowned Duke of Canterlot, a duchy that was granted to the descendants of the Platinums, the rulers of the Unicorn Kingdoms, in exchange for the crown and throne of the Principality of Equestria that was then granted to the two sisters. The title of Prince is a hold-over from the prior reign, and tacit acknowledgement of the line of succession in the unlikely event that the Princesses manage to die without an heir apparent. Given their apparent immortality, the Prince (or Princess) generally gets to live a life of quiet privilege and irresponsibility, serving as both a counterpoint to the benevolent reign of the Princesses, and a gentle reminder that things could be a lot worse.

7047569
Hmm... pretty close to mine.

7030302
I think "Prince Blueblood" is his name, not a title. Kinda like the musician formerly known as Prince, except that he's still known as Prince.

If he ever ascended and became an alicorn, he'd become "Princess Prince Blueblood", of course...

--Sweetie Belle

7030302 He may be a prince because of his relation to Celestia. It's possible that like Cadence he was taken in by her and became her nephew by extension.

7178912
Without becoming an alicorn? Sounds like there would be quite the story there...

7030302
Assuming his name isn't Prince (which may be the best head cannon for him ever ) I would imagine Celestia and Luna were adopted the by the royal family in canterlot (presumably after their birth parents died) either in gratitude for something they did, or as a excuse to keep them close.

7239591
So Blueblood would be a mortal cousin?

7239668
By virtue of the Sister's being adopted generations ago, yes.

Roughly a century before Luna lost herself, Celestia married again. Drab Tone was one of her aides up to that point. She couldn't quite continue her work in the palace for obvious reasons. They had foals. Celestia is, at heart, a mother. A mother to all, and a mother to her own. She likes to have a family. To be part of a family. Over the millennia, she had many spouses, and many children. Some bloodlines seeped into the dirt and just... ended. Others were cut short violently. She monitors them, keeps track of them. At this point, mostly out of curiosity. There are ponies living their lives in cities all over Equestria, unaware of 'royal blood' flowing through their veins.

Drab Tone was Celestia's last wife. After that came Luna's banishment, and her heart closed itself off in defense. That means a thousand years of lineage - minimum.

The Canterlot elite is a thing. The most wealthy ponies, the 'upper crust'. Many of them can trace their lineage back to ancient times, to some important ruler or aristocrat of old. Others just bought their way into these social circles. A random party, and a random pony remarks on the visual similarities between Celestia and Blueblood. Alcohol was had, everyone had a giggle. But the thought stuck with him. He was just the next bored generation of a wealthy family with much influence and little to do. But being of royal blood? Now wouldn't that be interesting.

He was chided by his family time and again for spending so much of the family's wealth on this obviously futile endeavor. All those historians and charlatans and scholars, pouring over every tome in every library in every city, bribing their way into the Royal Archives and whatever else.

They obviously shut up as soon as he succeeded.

Thanks to the ultimate superpower of 'being rich' - and the hard, laborious work of many ponies way smarter than him -, he could now prove his ancestry. He actually was related to Princess Celestia herself. Sure, there were a couple of generations between them, but who's counting, right? So he marches up to the throne and demanded to be acknowledged.

Celestia monitored her bloodlines. She had noticed him starting to dig. And for all she could tell, he was not a pony with decent character. That he would find something was unfortunate. Maybe she should have hindered his progress after all. That he would march straight up to her and start demanding things, that came as less of a surprise. And she denied.

More money was spent. On lawyers, this time. He would drag the Princess herself to Court if he had to!

And she was so very tired of his antics. What difference would it really make, giving in? He would boast about his 'accomplishments', others would be in awe. It would not make him a better pony. It could hardly make him a worse one, either. So she relented. The nobility gained an insufferable 'prince', Blueblood was happy and life went on, with little change.

Rarity would have a word or ninety to say about that, but hey - she doesn't know.

7689136
Certainly a unique take.

Roughly a century before Luna lost herself, Celestia married again. Drab Tone was one of her aides up to that point. She couldn't quite continue her work in the palace for obvious reasons. They had foals.

I feel there is a further answer needed here, but I should probably come up with and post an actual question for it first.

Rarity would have a word or ninety to say about that, but hey - she doesn't know.

Not in the Canterlot gossip rags she reads, eh?

7689190

Certainly a unique take.

Is it? I'm a firm believer that, with so many years of people having ideas, it has become hard, if not impossible, to actually have 'new' ideas. I would have betted that someone had thought of that already. Eh. Then again. It's still likely.

Not in the Canterlot gossip rags she reads, eh?

I just don't think Celestia ever shared with anypony just how little attention Blueblood's triumphant march to victory over her was worth. In her eyes, anyway. Rarity would most likely fume, hearing that. Because she would most certainly think that he would be less insufferable without that title.
He would, in fact, not be.

7689212
It can still happen, now and again. Especially if many just follow in the footsteps of popular takes.