There is a kind of denial we cling to when it comes to wild creatures in captivity. As if the smiling, performing orcas of SeaWorld are content to trade freedom for fish and applause. “Blackfish,” director Gabriela Cowperthwaite’s excellent new documentary, should change that. The film’s focus is on SeaWorld’s Tilikum, whose attack on veteran trainer Dawn Brancheau led to her horrific death in front of a theme-park audience (most of that footage isn’t shown). SeaWorld representatives, who declined Cowperthwaite’s interview overtures, have slammed the finished film as inaccurate and misleading. But “Blackfish” puts forth a powerful case for leaving these intelligent mammals in the wild. The story is built out of extraordinary footage of orcas, in the wild and in captivity, as well as experts and former SeaWorld trainers, who knew Brancheau and Tilikum. The film’s central debate circles around what exactly prompted the attack — natural aggression or captivity. An Occupational Safety & Health Administration case against SeaWorld is damning too, but the film’s power resides in its central truth — that Tilikum could not have killed Brancheau if he were free in the ocean.