The blood in the title refers to the blood of two demi-gods that must be spilled before Gaea, the earth-mother, the vicious, vindictive ancient proto-goddess can arise and essentially wipe all life off the planet. She's kind of ticked at having been put down by several generations of her offspring and if Momma ain't happy, ain't nobody happy.
Percy and Annabeth and Jason and Piper and the rest of their team on the Argo II face one fast-paced perilous mission after another, while Reyna and Nico and Coach Hedge are scrambling to transport the recovered statue of Athena to Camp Half-Blood before the Roman demi-gods from Camp Jupiter can attack. It seems some misguided egotistical self-proclaimed prophet believes that if the Romans wipe out their Greek cousins, then the big mental/emotional problems the gods have, constantly torn between their Greek and Roman personalities will settle down, and then they'll be able to deal with Gaea. Well, Octavian is an idiot, as is proven by his well-deserved demise. Unfortunately, it appears that one of our seven heroes is taken out at the same time ... maybe ... prophecies aren't exactly written in stone, y'know.
Whew! Lots going on, and I have to admit I was sometimes lost, because it has been so long since I read the last book in the series, that I forgot some of the adventures referred to. Major fun. One of those books you need to set aside huge chunks of time, so you can just keep reading. Satisfying ending. Although I must admit, I wouldn't mind more adventures. Although, just like on Stargate ... how many times can they save the world before it gets kind of ... redundant?