Just as Moon Called dealt with werewolves, so is Blood Bound focused on the vampires of the Tri-Cities. We get a lot of insight into their biology, how their society is structured, and what motivates them. Patricia Briggs has a thoroughly unsympathetic take on the species with individuals possessing quirky personalities but all of them being fundamentally evil.
Blood Bound's plot is interesting as it deals with a "chimera" monster. In this case, it's a sorcerer/demon/vampire. The actual mechanics aren't that difficult: a demon possesses a sorcerer and gets turned into a vampire. However, it's still something which has the potential to be silly. Demons in Mercy Thompson are mindless forces of destruction versus calculating corruptors. That hurts the book a bit, IMHO, and makes it purely a matter of finding and destroying it.
The big conflict is Mercy Thompson, mechanic and coyote shifter, is caught between trying to destroy the monster as well as not offending the local vampire seethe. The vampires of the Tri-Cities are powerful enough to stand against the local werewolves and a war could spread across the world. She has permission to destroy the demon-possessed vampire, Littleton, but also is aware the local undead would love to keep him as a weapon.
One of the things I liked about this book is it deconstructs the Friendly Neighborhood Vampire (see TV tropes). Stephen is a man who doesn't want to kill, doeshis best to feed from willing victims, and is an ally of our heroine but he's STILL evil. Vampirism feels more like a dreadful curse this way and poor Mercedes Thompson is stuck with the fact he loves her while she's repulsed by his actions. Too often romances with "bad boys" gloss over the dark side or reduce it to a generic surly attitude. No, Stephen is a man who has killed innocent people in the past and will do it in the future but desperately wishes he might not do such things. Mercy wishes she could say Stephen isn't an evil vampire but she'd be lying to herself.
The love triangle in the book is continued with Samuel and Adam competing for Mercy's affections. Much to my surprise, the second book heavily implies who the "final choice" is going to be and doens't really attempt to string out the story longer than it has to be. I appreciated this even as I also felt there wasn't that much to differentiate the two love interests to begin with. They're both big, domineering werewolves.
Mercy Thompson is an incredibly well-rounded character who is snarky, fun, and cheerful but also someone who takes her encounters with monsters seriously. Mercy is frightened of the vampires who live in the Tri-Cities versus being an unstoppable badass. That makes her more relatable and a protagonist I enjoyed following. Watching her try and figure out a way to balance her own safety with stopping the demon-possessed vampire is a lot more interesting than a hero who is boldly ready to give their life to destroy evil.
Are there flaws? Yes, basically, the problem with the book is also one of its strengths. Vampires are unable to be anything but evil by the "rules" of the setting so they are limited in the kind of characters they can be. Stephen is interesting because he tries not to be evil but everyone else is just a different kind of monster. As mentioned, the possessed vampire Littleton has almost no personality other than "muahahaha", which makes him a less interesting antagonist than the previous book's villain who just wanted to save his father's life. Still, overall, none of this impacted my desire to continue the series even if I liked Moon Called more.