What've we got on tap? How about some nice fresh blood?
While we're not blood-drinkers ourselves, we can appreciate the associations of blood in Marked. Blood appears in connection with intimacy, pleasure, and sexuality.
When Zoey scratches Heath, she can smell the blood on him: "I could smell it—it was like the wine, only better, worlds better. The scent of it wrapped around me and made the hair on my arms rise" (17.108). That, darlings, shows just how charged and electrifying blood is for Zoey.
Then she actually drinks a little of the blood, leading to this rather erotic description: "This time I leaned into him and touched my tongue to his lip, taking the drop of blood into my mouth where it exploded—heat, sensation, and a rush of pleasure I'd never known" (17.112). Whoa. That sounds pretty freaking intense, right? Can you see where we might think bloodlust is a stand in for other kinds of lust? Of the wink-wink, nudge-nudge variety?
The way Zoey describes how blood makes her feel definitely gives us the vibe that loss of control is a big tie-in. At one point she described the feeling as: "Dizzy and kinda weak-kneed? Like you're drunk, only you're not" (18.97). When blood's in the mix, then, it overpowers Zoey, taking control just like alcohol.
Zoey is freaked out by her new attraction to blood, just as she's freaked out by her new feelings of sexuality. Like yeah, she's made out in the past, but that's pretty much it—and now suddenly, she wants to explore more, which is scary for her, and potentially leads to her feeling out of control.
The bloodlust, then, is new and scary, like so many other parts of Zoey's life right now. So with blood serving as a metaphor for sexual intimacy, and bloodlust serving as a metaphor for how out of control she feels in her life, Zoey and her interactions with blood serve as markers for her general journey through coming of age as well as—bad joke alert—coming of vampire (hey, we warned you).