In music’s metal subculture, the late singer Ronnie James Dio may still count as the genre’s most widely beloved figure, which makes him ripe for a documentary. He was almost completely uncontroversial, which doesn’t necessarily bode as well for such a treatment. The dude who popularized the so-called “devil horns” hand gesture was no demon. He just played one on MTV, or at least enjoyed dragging out the sinister imagery, even if in in real life he came off as a friendly upstate New York guy who’d made it big with a penchant for vaguely mystic imagery and a mountain-king-sized voice that could fill the biggest halls.

“Dio: Dreamers Never Die,” the latest in a series of effective, mostly unpretentious rock docs produced by BMG, doesn’t present its subject as a particularly tortured or even complicated guy. It may be the most drugless documentary ever made about a preeminent hard-rock figure (although a line of coke does make a cameo appearance in the archival footage). But Dio had just enough curious career turns to justify the length of the movie that’s been made about him. Don Argott and Demian Fenton’s film belongs more to the realm of fan service than crossover pic, but it’ll attract curious souls from other corners of the rock world who don’t mind the devil being in two hours’ worth of details.

After making his name in the 1970s in two other bands, Elf and Ritchie Blackmore’s Rainbow, the singer really became the toast of the metal world when asked by Black Sabbath to replace the just-fired Ozzy Osbourne in 1979, for what wound up being a short but essential chapter in his career. Sabbath’s other members wanted someone more stable than Ozzy — to put it in understatement-of-the-decade terms — and they sure got that in Dio, the utter Gallant to Osbourne’s Goofus. A clown price was replaced by a gentleman frontman who, after all was screamed and done, would rather retire to his manor than close down the pubs. It didn’t hurt that Dio was probably the most naturally gifted singer of his chosen genre, at least if you prefer your tales of dragons and destiny belted operatically from on high, not growled at sulfur level.

Dio, who died abruptly of late-diagnosed cancer in 2010, is generally remembered as one of rock’s nicest superstars. That didn’t preclude him from having a very healthy ego, so there is a lot of band turnover history to cover along the way, dating all the way back to the ’50s. After a present-day introduction to wife and manager Wendy Dio (also the film’s executive producer), “Dreamers Never Die” jumps back in time for its first and most delightful surprise: the reveal that before he was a god, he was the square-looking trumpet player and slick crooner in a series of post-rockabilly bands, then a slightly counterculture guy, before the pre-metal “heavy rock” of Deep Purple turned his head. Seeing and hearing him in these early groups is kind of like watching the early, “historical” footage of Spinal Tap navigating the ’60s as flower children.

This won’t be the last time you think of “This Is Spinal Tap.” It’s difficult to imagine that that comedy’s creators weren’t influenced at least a little by Dio’s act, even if it wasn’t until later in the ’80s that he made it a nightly habit to wield a prop sword and slay a 13-foot, laser-eyed dragon on arena stages each night.

The least likely to find any of it a little silly is super-fan Jack Black, who gave Dio, his ultimate hero, a delicious musical-comedy cameo in 2006’s “Tenacious D in the Pick of Destiny” and gets to gushing here. Any media voices heard from in the film are also adoring metalheads, like hard-rock radio jock Eddie Trunk, who provides a telling comment late in the film when he’s discussing Dio’s prop-filled stage show: “Every time Ronnie slew that dragon, that was what heavy metal is all about: authenticity.”

That comment feels like it has to be the punchline of a joke, but of course, to Trunk, and to the devoted filmmakers, it isn’t. By the documentary’s partly convincing logic, Dio’s sword-and-sorcery inclinations really were the ultimate exemplar of some kind of cred. At one interesting turn earlier in the film, Dio is remembered as holding his ground in the late ’70s when then-boss Ritchie Blackmore panicked at Rainbow’s fading commercial prospects and suggested, “Do you think we should write some songs about relationships?” That’d be a reasonable request in any other rock genre, but Dio split from that band rather than subject himself to anything so low and unmystical as a love song.

Dio’s flock loved him because he imparted a feeling of power, not puppy love. Ironically, that ability to make young misfits feel plugged in to something other than sex may be something he had in common with the guy who effectively knocked his brand of rock off the charts for good, Kurt Cobain … even if the dog-level octaves, grandiosity and prop swords had to go.