It’s a weird miracle that Johnny Cash and his primitive twosome banged out music that still feels so full and vital today. And it’s a weird miracle that My Father and the Man in Black is itself full and vital, despite throwing off all sorts of vanity-project warning signs: It’s directed by a first-timer with a personal stake in the story. It tells much of that story through green-screened reenactments in which actors play the father of that director and no one less a personage than Cash himself, that hopped-up oak of a man. It even opens with a conversation between the director at age seven and his father. But it has the heart, and it has the blood, and by the time childhood chatter is played back again, feeling is soaked through it like the sweat in Cash’s guitar strap. Writer-director Jonathan Holiff is the not-quite-estranged son of Saul Holiff, Cash’s manager during what could be called the interesting years: the pill-fueled ’60s, the triumphs at Folsom and San Quentin, the wedding to June Carter, and the conversion, in the early ’70s, when Cash goes all in for Jesus, losing his TV show, many fans, and Saul, a Canadian Jew who is unwilling to tell his client “I accept the divinity of Jesus Christ.” One long phone call concerning Cash’s ill-fated Jesus movie is almost painful. The great man, you’ll hear, could also be pushy and insecure, and he wasn’t entirely free of the suspicions common to other white folks born in Arkansas in 1932. By the end, it’s as much about family as it is about showbiz craziness.