The Indigo Spell (Bloodlines, #3)

Sydney Sage is an Alchemist, one of a group of humans who dabble in magic and serve to bridge the worlds of humans and vampires. They protect vampire secrets--and human lives."
In the aftermath of a forbidden moment that rocked Sydney to her core, she finds herself struggling to draw the line between her Alchemist teachings and what her heart is urging her to do. Then she meets alluring, rebellious Marcus Finch--a former Alchemist who escaped against all odds, and is now on the run. Marcus wants to teach Sydney the secrets he claims the Alchemists are hiding from her. But as he pushes her to rebel against the people who raised her, Sydney finds that breaking free is harder than she thought. There is an old and mysterious magic rooted deeply within her. And as she searches for an evil magic user targeting powerful young witches, she realizes that her only hope is to embrace her magical blood--or else she might be next.