Robert Sheff, a composer and pianist who worked under the name “Blue” Gene Tyranny as a solo performer and a collaborator with artists including Iggy Pop, the composer Robert Ashley and the jazz composer and arranger Carla Bley, died on Dec. 12 in hospice care in Long Island City, Queens. He was 75.

The cause was complications of diabetes, Tommy McCutchon, the founder of the record label Unseen Worlds, which released several albums by Mr. Tyranny, said in an email.

His memorable pseudonym, coined during his brief stint with Iggy and the Stooges, was derived partly from Jean, his adoptive mother’s middle name. It also referred to what he called “the tyranny of the genes” — a predisposition to being “strongly overcome by emotion,” he said in “Just for the Record: Conversations With and About ‘Blue’ Gene Tyranny,” a documentary film directed by David Bernabo released in September.

Music, Mr. Tyranny explained in the film, was a source of solace, but also a means “of deeply informing myself that there’s another world. Music is my way of being in the world.”

A master at the keyboard and an eclectic composer who deftly balanced conceptual rigor with breezy pop sounds, Mr. Tyranny was active in modern music as early as his teenage years.

From curating contemporary-music concerts in high school, he went on to participate in the groundbreaking and influential Once Festival of New Music in Ann Arbor, Mich., during the 1960s. He taught classes and worked as a recording-studio technician at Mills College, an experimental-music hotbed in Oakland, Calif., from 1971 to 1982. Arriving in New York City in 1983, Mr. Tyranny worked with Mr. Ashley, Laurie Anderson and Peter Gordon’s Love of Life Orchestra, while also composing his own works.

Mr. Tyranny, who had been living in Long Island City since 2002, is survived by a brother, Richard Sheff, and three half siblings, William Gantic Jr., Vickie Murray and Justa Calvin.

He was born Joseph Gantic to William and Eleanor Gantic on Jan. 1, 1945, in San Antonio. When Mr. Gantic, an Army paratrooper, was reported missing in action in Southeast Asia during World War II, Mr. Tyranny related in “Just for the Record,” his wife gave up their infant child for adoption.

He was adopted 11 months later by Meyer and Dorothy Jean Sheff, who ran a clothing shop in downtown San Antonio, and renamed Robert Nathan Sheff. He began piano studies early in his childhood and took his first composition lessons at 11. By high school, he was performing avant-garde works by composers like Charles Ives and John Cage in an experimental-music series he jointly curated with the composer Philip Krumm at the McNay Art Institute in San Antonio.

Invited by the Juilliard School to audition as a performance major, he demurred, insisting even then on being viewed as a composer. Instead he went to Ann Arbor, where he lived and worked from 1962 to 1971 and participated in the Once Festival. Mr. Tyranny’s works from this period, like “Ballad” (1960) and “Diotima” (1963), were abstract and fidgety, chiefly concerned with timbral contrast.

In 1965, Mr. Tyranny helped found the Prime Movers Blues Band, whose drummer, James Osterberg Jr., would achieve fame as the proto-punk singer-songwriter Iggy Pop. Another founder, Michael Erlewine, later created AllMusic, which became a popular reference website to which Mr. Tyranny contributed, occasionally writing about his own work.

In the late 1960s, Mr. Osterberg transformed himself into Iggy Pop and formed the Stooges. After releasing the album “Raw Power” in 1973, he invited his former bandmate to join him on tour. Mr. Tyranny accepted, performing with red LED lights woven into his hair.

He also played in the bands of jazz composers like Bill Dixon and Ms. Bley, and in 1976 explored the intersections of contemporary classical music and rock with Mr. Gordon in a groundbreaking concert series in Berkeley, Calif., documented on a 2019 Unseen Worlds release, “Trust in Rock.”

An association with Mr. Ashley, whom Mr. Tyranny had met in Ann Arbor and then followed to Mills College, flourished into a close, enduring collaboration. Mr. Tyranny’s best-known work likely was the role he created in “Perfect Lives (Private Parts)” (1976-83), Mr. Ashley’s landmark opera, conceived and eventually presented as a television series: Buddy, the World’s Greatest Piano Player. Their relationship was deeply collaborative. Presented by Mr. Ashley with a blueprint indicating keys and metric structures, Mr. Tyranny filled in harmonies and supplied playfully ornate piano writing.

“Blue and Bob had this symbiotic relationship from back in Ann Arbor,” Mr. Gordon, who also participated in the creation of “Perfect Lives,” said in a phone interview. “The character Buddy is like the avatar for the music of ‘Blue’ Gene.”

“What we commonly recognize as music in ‘Perfect Lives’ was ‘Blue’ Gene’s,” Mr. Gordon explained, “but the overall composition was Bob’s.” Mr. Tyranny would contribute in different ways to later Ashley operas, including “Dust” (1998) and “Celestial Excursions” (2003).

In his own music, much of which he recorded for the Lovely Music label, Mr. Tyranny moved from early efforts with graphic notation and magnetic tape to compositions that drew from popular styles. Some selections on his debut solo album, “Out of the Blue” (1978), like “Leading a Double Life,” were essentially pop songs. “A Letter From Home,” which closed that album, mixed found sounds and dreamy keyboards with an epistolary text, spoken and sung, ranging from the mundane to the philosophical.

He worked extensively with electronics and labored throughout the 1990s on “The Driver’s Son,” which he termed an “audio storyboard.” A realization of that piece, a questing monodrama set to lush timbres and bubbly rhythms, will be included in “Degrees of Freedom Found,” a six-CD boxed set of unreleased Tyranny recordings due on Unseen Worlds in the spring. Mr. Tyranny, who lost his eyesight in 2009 and gave up performing after 2016, helped to compile the set, hoping to give his disparate canon a coherent shape.

Mr. Tyranny’s compositions divided critical response. “To this taste, Mr. Tyranny’s work too often skirts the trivial,” John Rockwell wrote in a 1987 New York Times review. But Ben Ratliff, in a 2012 Times review of the last new recording issued during Mr. Tyranny’s life, “Detours,” offered a different view: “Mr. Sheff represents a lot of different American energies.”

He added, “He does not stint on beautiful things — major arpeggios, soul-chord progressions, lines that flow and breathe — and his keyboard touch is rounded and gorgeous, a feeling you remember.”