The whole time I was reading Half-Blood, I was reminded of Vampire Academy by Richelle Mead. It's Vampire Academy, but with a Greek mythology twist. 
 I loved the idea of Half-Blood. The Hematoi, and pures and halfs...I love how the society is structured. And I loved the idea of the Apollyon...and the rare occurrence of 2 Apollyons. Of course, Alex happens to be the 2nd Apollyon. 
 Half-Blood just didn't work for me though, and I think it's because I couldn't stop comparing it to Vampire Academy. The 2 books are similar enough that I couldn't really enjoy Half- Blood. 
 The characters were just okay. Alex just annoyed me, with her need to take care of things, even when told not to...and even thought she needed a lot more training. The only character that really stood out was Seth, who's the first Apollyon. His abilities are definitely mysterious, but also interesting. 
 Armentrout does keep you interested, and the novel moves along pretty well. I wasn't bored while reading it, which is always a plus. I liked the references to Greek mythology. 
 Overall, I have to give it a 2 out of 5. It was okay, and while I liked the concept, I was too reminded of VA to really enjoy it.