There’s been a flood of throwback tween terror on movie screens and streaming services for the past few years: from “Stranger Things” to the newer “It” iterations, it’s been a banner era for floppy-haired pubescents in ringer tees. Scott Derrickson’s “The Black Phone” fits neatly into the subgenre, but this incredibly dark kiddie kidnap horror film just hits different with a hard-R rating, going for the jugular with a surprising extremity of violence, plus a tone that wobbles between the bleak and the buffoonish. Adapted from a short story by Joe Hill (the son of Stephen King), “The Black Phon…