Entirely depends on the kid.
17yo ds loathes horror. I got him to download Perfume for me the other day, as I'm teaching the novel next year. Suggested we could watch it together , he noped the absolute fuck out of that - I wouldn't even class it as horror. Oddly, he loves The Shining after watching a documentary about it. Deconstructing & analysing horror films is fine apparently, but watching them 'cold' is frightening & he hates it.
15yo dd1 adores scary stuff. She'd love Blair Witch. In fact, good suggestion - we have a mother & daughter creepy film night occasionally, & it's years since I watched it. We spent a few cheery months in lockdown ripping the piss out of The Walking Dead together.
13yo dd2 - not a chance. She'd get bored long before she got creeped out, though.
I would say that with Blair Witch, either it scares you silly or you don't get it, though. Not a great choice either for a horror hater or a teen who doesn't already enjoy horror.