Saw your query ask and I thought I'd try to help out. Hope this helps!
Sam Hain (I see what you did there and it made me snort/laugh in the best way possible) deals with things that shouldn't exist: he specializes in monsters who prey on people.(This sounds like an episode of Supernatural and that makes me happy).
So he's out of his depth when a desperate mother comes to him as a last hope of finding her missing son.(Why would he be out of his depth? How does he usually catch cases? It seems like this would be a relatively common way? And why does she come to him? Is he known as the Supernatural Guru? Or is she just running around with missing fliers, stapling them to anything and anyone who stands still long enough?) The boy, Bryan, has been missing for months. The police have found nothing. Whoever took him didn't leave behind a single shred of evidence. (How about: The lack of evidence left behind by his kidnapper leaves the police stumped. I just felt like these two sentences should be condensed) However, after Sam does some digging, it turns out Bryan isn't the first child to go missing in Lurnely, either. At least a dozen other children have vanished in the last few decades. (This sentence feels like a jump forward from the last one, like there needs to be something that takes us from “There is no hope and no clues” to “Okay, now he’s part of a pattern”).
Diving deeper, Sam learns these disappearances started after a disaster in the mine, the historical center of the town's economy
for most of its history. He also learns Bryan encountered a black-eyed girl before he disappeared.(From who? How? Something about this seems off.) A girl who looked like the child who vanished before him.(These two sentences seem like they should go together too, but they both seem off.... Forced. Like I see what you’re doing and why they need to be included, but they kind of come out of your voice and feel very… rote. What about: A local tells him (or whatever happened) that the last time Bryan was seen was in the company of a black-eyed girl who bore a striking resemblance to the last child who vanished.) Convinced the mine and children are connected, Sam descends into its depths, where he discovers a monster unlike any he has ever encountered before.
The monster infects him, turning more than just his eyes black. With his body succumbing to this darkness, he has only hours to find a way to kill the monster, free the children of its control, and save himself before he joins the creature in the shadows. (I might be wrong, but this whole section feels like it’s giving too much away for a query. Or maybe that it’s jumping in and explaining a lot of things way too fast. Something else I notice is that we have very little sense of what this creature is, does, wants, etc. Which is fine if you’re going for an air of mystery, but you’re undermining that with the “this is what happens” vibe of this. If it were me, I’d leave it out of the query, but definitely include a more fleshed out version in the synopsis. Side note-- a lot of the advice posts and "how to" guides suggest that these be done in two paragraphs. I know there's a bunch of conflicting advice, but I thought it worth a mention.)
At 79,000 words BLACK EYES is an adult urban fantasy novel. (NICE!)
Overall notes: Cool story idea! I'm a character person, so I'd love to see a little more of Sam fleshed out because I don't really get a sense of who he is or why he's doing this (which is SUUPER hard to convey in a query and I completely understand your pain). Anyway, I'd totally pick this book up. Kinda reminds me a little of mine :) Good luck!