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We all know the first line of a query is your "hook." I call the last line the "sinker." You want it to punch them in the face, in a nice, friendly kind of way that makes them unable to forget you after having read the 300 other queries in their inbox.
If you're looking for query advice, but are slightly intimidated by my claws, blade, or just my rolling googly-eyes, check out the query critique boards over at AgentQueryConnect. This is where I got my start, with advice from people smarter than me. Don't be afraid to ask for help with the most critical first step of your writing journey - the query. My comments appear in green.
There is no such thing as true immortality. Eternity is a long time to make enemies, and Carla Dubrov knows that sooner or later, everyone gets killed. I think your second line is a good, voice-filled hook. Maybe consider putting your first one last?
To survive, Carla has taken the lives of many immortals, but the one that will forever haunt her was not taken—it was given. "taken" echo -- rephrase and you'll be fine. Anthony, the man she loved, Calling Anthony a "man" and Jason a "boy" makes me wonder about Carla's own age since it's not stated. And that's fine.. but the fact that she was in love with a "man" and this is YA makes me feel a little icky sacrificed himself for her. Now, two hundred years later, she can finally repay the debt by saving his brother, Jason. The boy’s reckless search for Anthony’s awk. phrasing here with the possessives killers has attracted attention that is certain to turn deadly. No one knew Anthony had a living brother—not even Carla—and for Jason’s own sake, that secret should have never been revealed.
When two of Jason’s friends are compelled to commit suicide, it becomes clear that Carla’s enemies—the same enemies who killed Anthony I think that's assumed—have found them. Carla’s plans to keep him alive are frustrated by the fact that Jason wants nothing to do with her; he is convinced she was involved in Anthony’s death, and he accuses her of luring the killer to his hometown. All Carla can do to protect him is try to find and stop the killer before he or she try "they" gets to Jason. Despite all of her efforts, her adversary is always one step ahead. As the deadly game unfolds, it becomes clear that Carla is the ultimate target. Jason is simply the means to making her suffer.
The instincts that have kept her alive for so long are telling her to run, but she has no guarantees that the killer won’t stop to finish Jason off before following her. She can’t leave him behind. But staying also has consequences, one of which could very well be her own death.
SHADES OF DARKNESS: THE LIGHT is a young adult contemporary fantasy novel, complete at 83,000 words.
Not bad at all, but you should clear up the ages here. You don't necessarily have to state them flat-out, but you call Jason a "boy" and then make no allusion whatsoever to a romance b/w himself and Carla... which is totally fine. But it definitely makes me wonder if he's a child, or if he's a teen - and if so, is there a romance that isn't made clear in the query?
Also, the colon in your title makes this look like the first in a series. If that's the case, you need to state that here.