This movie doesn't have a lot going on, really - six total characters, and really, only five get all that much screen-time. It felt like a cheaper version of Eden Lake, more or less.
While at first I didn't know where the story was going, once the catalyst to the action occurred, everything afterward was pretty predictable. The gore was decent, more on the realistic side as opposed to a splatter film.
What was really the strong point of the movie was the friendship, somewhat strained, between the three main female friends. While the dialogue can sometimes feel like a bit much, I thought it worked out well, and the actresses all did a decently good job (Kate Bosworth being my favorite, as I also enjoyed her performance in 2008's crime/drama movie 21). For most of the beginning, it felt like a real friendship, and to me, they seemed to talk like real people, which isn't something that is always seen in movies.
Past a certain point, though, things drag a bit, which is saying something, as the film's under 80 minutes. It starts out decently strong, but peters out about twenty minutes in or so. All-in-all, Black Rock's not a bad movie, but in the end, it feels rather more average than I'd have preferred. An okay viewing the first time around, but I wouldn't go out of my way to see it again. 6.5/10 (rounded down to 6/10 to fit site's format).