Symbolism, Imagery, Allegory
Blood plays an important role in the lives of vampires (duh). They need fresh blood to be functional, though apparently a guinea pig's worth of blood will tide them over for a day at a time. So in one sense, blood = food, even though it's still not perceived as particularly appetizing. Nina describes the act of feeding as a "repugnant procedure" (6.66), after all. This distaste matters: For Nina and the others, being a vampire comes with serious downsides, one of which is the very thing they need to consume in order to stay alive.
And then there's blooding, which according to Nina is "that moment when, as a vampire, you first smell fresh human blood and have to fight the urge to bite someone" (3.10). This is an abstract concept to Nina because she's managed to avoid exposure to human blood for the few decades of her vampiric existence. But thanks to thing going poorly once they reach Cobar, Nina's exposed to human (or rather werewolf) blood—and the way Nina describes this encounter is pretty intense:
[…] the smell hit me, practically knocking me off my feet. It was like being struck by a train; there's no other way of describing the sensation. […]
If you've ever been on the verge of drowning, you'll understand the desperate, frantic, primitive need that overwhelmed me. If you've ever had the chicken pox, and experienced the urge to scratch—if you've ever had a cold, and felt a cough coming on—if you've ever suffered from a bursting bladder, than you'll have an inkling of what it was like. My teeth were aching. My mouth was parched. […] My peripheral vision faded as I zeroed in on the nearest person. The nearest throat. (11.14-15)
Whoa, dude—that's intense. Being exposed to fresh blood feels like an overwhelming need that takes over all of Nina's senses and pretty much puts her rational brain to sleep. So in this instance, blood represents a paralyzing need for sustenance as well as the infection trying to hijack Nina's humanity. The fact that she successfully fights it (with Dave's help) shows Nina clinging fiercely to her humanity and refusing to let the vampiric side of her win.
So blood, Shmoopers, represents a double-edged sword to vampires: On the one hand, it's food, and as such, a normal part of their daily (nightly?) existence. But on the other hand, the lure of blood represents a serious challenge to a vampire's humanity. It ain't easy being a vamp.