I totally get it as a comment on rape culture, psychological fallout of war and a statement that women will stand up and fight their longtime oppressors with all they got. But as a film, it fails miserably. I'm sorry for Aselton and Bell, who are both great performers, but the little script they had was terribly ham-fisted and the improv angle reeeeaaallly didn't work for them here.
Things you have to overlook include horrid dialogue (mainly from the men) and unlikable, one-dimensional characters. Despite this, the cinematography was sharp and as an exercise in tension it was successful. It rarely dips into new waters, and sticks to the familiar hunter-becomes-the-hunted routine of survival thrillers, but even then it was worth the viewing. Seems cocky to compare itself to Deliverance, though.
Bosworth - spoiler alert! - gets out while the getting is good. A respectable mumblecore set-up doesn't build to sequences of suspense/horror, but one instance of melee violence. Aselton and Duplass load Rock up with maladroit fodder for a Feminist reading (...or to tidily dodge charges of misogyny). Transgressive horror scenarios (rape/revenge, "most dangerous game") have never seemed so tame and, frankly, boring.
Neither as clever as the recent Eden Lake, nor as weird as Deliverance, Black Rock still manages to make the heart pound during the last few minutes. It has many flaws, and if you're inured (like I) to annoying female horror characters then you won't mind the protagonists, but it's still entertaining enough and worthy of one watch. It's closest to is I Spit On Your Grave with a hint of The Most Dangerous Game...