“Black, White + Gray: A Portrait of Sam Wagstaff and Robert Mapplethorpe,” a new documentary about the late curator Sam Wagstaff, calls him “totally forgotten.” This epithet is not entirely accurate, though if he is remembered today, it is too often solely for being the longtime partner of the photographer Robert Mapplethorpe. But Wagstaff didn’t just cleave to a photographer; he was a lover of photography. Indeed, he became one of the most important and perspicacious collectors of photography ever. Although subtitled “A Portrait of Sam Wagstaff and Robert Mapplethorpe,” the film, by first-time director James Crump, is rightly a biographical sketch of the collector, not a double portrait. Minor inaccuracies like this one, as well as some problems of balance, at times distract from what is otherwise a delightful and informative documentary.

Raised by his Polish émigré mother on Central Park South (we’re not told what happened to the father), Wagstaff (1921–89) was the scion of a patrician family. They apparently once owned the farmland upon which the Metropolitan Museum now sits. He attended the Hotchkiss School and Yale and served in the Navy during World War II. In the 1950s, he went into advertising, a “glamorous” profession in those years, says celebrity journalist Dominick Dunne, who knew him at the time. Tall and extremely handsome, Wagstaff was, according to Mr. Dunne, “a New York deb’s delight.” If the film lingers a bit heavily on such ’50s contrivances as advertising, square families, and the like, it does establish Wagstaff as a man who, for a while, conformed to the era’s expectations. “Sam Wagstaff was no sissy,” a female friend exclaims.

He eventually left the corporate world to study art history at NYU, and it was then that he began accepting himself as a gay man. By the early ’60s, he had also blossomed into a cutting-edge curator and collector of art (though not yet of photography), forming part of a trio — with Frank O’Hara and Henry Geldzahler — of young curators who led the march from Abstract Expressionism to Pop and Minimalism. At the Wadsworth Atheneum Museum of Art, in 1964, he mounted “Black, White, and Gray” — from which the film takes its name — an important and forward-looking exhibition, though arguably not, as the film would have it, the first show of Minimalist art. No one, however, disputes Wagstaff’s contributions as a curator. He gave the artist Tony Smith his first solo museum show and supported such artists as Richard Tuttle, James Lee Byars, Agnes Martin, and Andy Warhol. And like Warhol, Wagstaff reveled in the ’60s nexus of art, celebrity, and fashion. Also, we’re told, “he loved drugs.”

Still, Wagstaff’s exit from the institutional art world demonstrates both his integrity and wit. While working at the Detroit Institute of Art (from 1968–71, information we’re not given, as Mr. Crump tends to be hazy on dates), he presented Michael Heizer’s “Dragged Mass Geometric,” which entailed lugging a huge cement block across the museum’s grounds. The grounds were destroyed, and Wagstaff was forced to pay for the damages. “A triumph for manicured lawns over fine art,” he said, and left the museum.

Shortly thereafter, when he was 51, Wagstaff met the 26-year-old Robert Mapplethorpe (1946–89), who came from the working-class neighborhood of Floral Park, Queens, and had studied painting at the Pratt Institute in Brooklyn. Around this time, Wagstaff sold off his collection of avant-garde paintings —including, for example, Jackson Pollock’s “The Deep” — and, in 1973, started buying 19th-century photographs. As to who influenced whom in photography, the record is not clear: The film suggests that Wagstaff urged Mapplethorpe, who was making jewelry when the two met, to become a photographer, though the young artist certainly had an effect on Wagstaff’s tastes.

The two men fell in love, yet several of the film’s interviewees argue somewhat nastily that the relationship benefited Mapplethorpe more than it did Wagstaff. The dealer Holly Solomon emphasizes Mapplethorpe’s manipulativeness, for example. Patti Smith, the musician who was close to both men and comments throughout the film, mildly agrees, noting, however, that “Sam was not fragile.” Access to the older man’s collection certainly nourished Mapplethorpe’s development as an artist. Some of these negative comments on the photographer feel tainted by homophobia. On the other hand, the writer John Richardson suggests that Mapplethorpe helped Wagstaff throw off the repression of his youth and become a happier person.

Wagstaff, it seems, was completely at ease with his collecting persona. The first person to recognize the aesthetic value of vernacular photography and among the first to collect anonymous photographs, he was a fearless connoisseur. “Where other people in the photography world bought names,” says a fellow collector, “Sam bought great pictures.” He gravitated toward “tough” images: medical and ethnographic photos, as well as the erotic male nudes of George Platt Lynes, pictures that had a profound impact on Mapplethorpe’s work. He also pioneered the market for photography (the film uncharitably characterizes this as “clever speculation”). In 1984, Wagstaff sold his collection to the J. Paul Getty Museum for $5 million, a record at the time. An inveterate collector, he next turned, improbably, to American silver.

Wagstaff died of AIDS in 1987, two years before Mapplethorpe succumbed to the same disease. If anything, “Black, White + Gray” feels brief: It could easily shoulder 20 minutes more material. Still, it’s a finely drawn portrait, and admirably explores all the shades of Wagstaff’s fascinating character.