When you go to summer camp, you meet friends who last a lifetime. It happened to Charlie Brown and it happened to Percy Jackson about nine books ago. In The Blood of Olympus, Percy Jackson and his pals go places they probably never dreamed of. Actually, they often have prophetic visions, so they probably did dream of these adventures, but still…they'd never have been able to do any of this by themselves.
The friendships between our Greek and Roman protagonists show the warring camps that cooperation is possible. They lead by example.
While a friendship might lead them into trouble—like Jason following Percy under the sea—it always gets them back out.
Gods love sacrifices. We're talking everything from A to V, where "A" is animals in honor of a god and "V" is virgins tossed into volcanoes. Some people even suggest that the Parthenon itself was the site of human sacrifice. But in The Blood of Olympus, the sacrifice needed is right in the title: blood. And the giants hope to spill it at the Parthenon, for old times' sake.
Leo makes the ultimate sacrifice, but he probably wouldn't were it not for the existence of the physician's cure.
Sometimes a small sacrifice is necessary—like giving up the cornucopia, for example—to prevent the loss of something larger.
You can't walk through any good fantasy story without tripping over a prophecy or two. Sybil Trelawney makes one in Harry Potter and the Order of the Phoenix. Eowyn fulfills one in The Return of the King. And there are more prophecies in The Blood of Olympus than you can shake a scepter of Diocletian at. Because it's a story rooted in Greek and Roman myth, the prophecies almost all have a tendency to come true. But since the characters are demigods—and not mere mortal pawns of the gods—they have at least a little agency when it comes to determining their own fate.
Greek and Roman tragedies are all about fate; that's why there are three of them personified.
Leo is able to both go with his fate and exercise his free will by sacrificing and subsequently resurrecting himself.
A hero is a person "who has shown unusual bravery or courage." The dictionary says so.
We've got a lot of heroes in The Blood of Olympus, and not just because pretty much every character is the spawn of a god. Being big and strong doesn't make someone a hero (remember the cowardly lion?). It's what a person decides to do with their strength—or other magical superpower of choice—that gives them the title.
The Blood of Olympus is the final book in the series, so pretty much every character has to demonstrate their bravest moments when the stakes are at their highest.
The gods themselves may be all powerful, but they're not brave. They wait until the very last minute to help the demigods defeat the giants.