BERLIN (Reuters) – They’ve been together for years, their marriage is harmonious and fulfilling. She is successful, he is content. Until one day an ex appears and everything is destroyed in an instant. The premise of “Both Sides of the Blade,” which premiered at the Berlin Film Festival on Saturday, may not be entirely original, but for director Claire Denis, her film is distinguished by its adult, unflinching approach to a menage-a-trois and a refusal to apportion blame. “That was the film’s angle,” she told a news conference. “We weren’t going to judge them. We weren’t going to reject them. …