Terence Blanchard has been in rehearsals, with pencil and paper at the ready, as he tailors his opera ahead of its New York premiere.A basement rehearsal room at the Metropolitan Opera was so packed recently that it began to resemble a sweltering boxing gym.In one corner, members of the Met’s music staff were grouped together like judges tallying punches as they looked down at their scores. Nearby, a drummer and pianist locked into a syncopated groove, following the beat of Yannick Nézet-Séguin, who was conducting while seated on an elevated platform.A phalanx of dancers rushed in to evoke an intense, collective workout regimen filled with balletic grace and pugilistic intensity. Those moves were choreographed by Camille A. Brown, who was close by, keeping an eye on every acrobatic feint. A former World Boxing Organization heavyweight champion paced the room, offering exhortations and encouragement.Supervising all this was the composer and trumpeter Terence Blanchard. He watched as his first opera, “Champion,” took shape ahead of its Met debut on Monday. (A Live in HD simulcast is planned for April 29.)After premiering at Opera Theater of St. Louis in 2013, “Champion” has played at the Washington National Opera and, scaled to a chamber-size orchestration, at SFJazz in San Francisco. But when this work — modeled on the life of the boxer Emile Griffith, and following Blanchard’s “Fire Shut Up in My Bones,” which arrived triumphantly at the Met in 2021 — opens in New York this week, it will be thoroughly revised and expanded to embody the composer’s recent thoughts about opera, as a form. To wit: in this latest version of “Champion” there are not only new arias (and new lines for supporting characters); what will be heard in New York this season also reflects Blanchard’s latest work when it comes to orchestral complexity and vocal elegance.Performers in “Champion” evoke the world of boxing in choreography by Camille A. Brown.Ike Edeani for The New York TimesBlanchard has been in “Champion” rehearsals, at the ready to revise his score as needed.Ike Edeani for The New York TimesThe opera tells the life story of Emile Griffith, who is depicted in two roles sung by Ryan Speedo Green and Eric Owens.Ike Edeani for The New York TimesFor example, during the rehearsal last month, the soprano Latonia Moore, as Griffith’s mother, was singing a rhythmically bumptious riff from the first act when she and Blanchard noticed that the phrase, as written, wasn’t sitting in the most powerful part of her range. “Hey, Mr. Living Composer,” she called out, in a teasing tone. “Could you rewrite this for me?”Blanchard got to work immediately, composing a new vocal part on a blank page of staff paper: a melodic line that could work atop the existing orchestral harmony. He took a photograph of the revision before passing it along.“I couldn’t believe that he just sat there right in the room and wrote it,” Moore said later. “I expected he would come in with it a few days later, OK? It was like, ‘No, here it is.’ Oh my God! And it was really good.”In an interview after a rehearsal, Blanchard explained how his flexibility — unusual in the world of opera, in which scores, like schedules, are set far in advance — was the result of some early, on-the-job training in his career as a jazz performer.“Art Blakey taught me years ago: The easiest thing to do is to write something nobody can play,” Blanchard recalled. “The magic comes in not just through the melody and the harmony, but who’s playing it.”“You can see she has a powerful voice,” he said of Moore. To him, the calculation was simple: He wanted to feature that voice in the strongest possible way. “So that’s what it’s gonna be changed to.”Blanchard, right, with Joshua Balan, a cast member.Ike Edeani for The New York TimesMoore’s role, as that of Griffith’s manipulative and sometimes absent mother, is hardly the only one to be subjected to extensive revisions. The bass-baritone Ryan Speedo Green — a standout in “Fire” and the star of “Champion” — said that when he first discussed this opera with Peter Gelb, the Met’s general manager, he felt that the role was a touch high for him.Gelb told him, “Speedo, That’s the beauty of having a living composer: Things can change,” Green recalled.“CHAMPION,” WITH A LIBRETTO BY MICHAEL CRISTOFER, TELLS Griffith’s tabloid-ready life story. Green sings Young Emile, while the veteran bass-baritone Eric Owens is cast as Old Emile, who lives in a nursing home on Long Island in the early 2000s. The boxer leaves the Virgin Islands for New York, then works in a hat factory before becoming a welterweight champ in the 1960s. In the ring with Benny Paret, Griffith unintentionally delivers blows that prove to be fatal, leaving Griffith anguished for years.“There’s this dream state that Emile is in,” Blanchard said, “because he’s dealing with dementia. There’s a combination of that harmony and that voicing, versus when it’s younger Emile. And chords moving; it goes back and forth. But it’s all story-driven, and it’s story-driven inside my language that I grew up listening to, as a jazz musician.”There is another thread in the opera, of Griffith’s journey from a straight-coded world to one of queerness. As a young man, in New York, he is drawn to gay bars and men while also excelling in the “man’s world” of boxing. The sports universe either doesn’t want to hear about queerness, or openly derides him for his sexual orientation.Just as Griffith navigates dramatic contrasts, so too does Blanchard’s score.The composer likes to talk about his love for Puccini — and you can hear some of that in Young Emile’s Act I aria “What Makes a Man a Man?” But in the boxing sequences, there’s a driving sense of muscular, post-bop jazz tumult. (As in “Fire,” the drummer Jeff Watts, known as Tain, leads a jazz combo embedded within the orchestra.) And there are some moments in which the fusion is well blended enough that no stylistic input seems to have the upper hand.Blanchard said that from his first visits to New York, starting in the spring of 1980, he took in a wide range of music. Although he was associated with traditionalist-minded players of New Orleans, he made a point of hearing the trio Air, which included the cutting-edge music of composer Henry Threadgill.“People were like, ‘Why are you going to that?’” Blanchard said. “And I’m like: ‘Bruh, because I’m trying to figure out what fits for me. I want to experience it all. Why limit myself, because you think I shouldn’t like this? Let me find out for myself.’”Those experiences pay off in “Champion.” In one of the early scenes at a gay bar, Blanchard writes sumptuous orchestral music — a cousin of sorts to the bluesy music heard in a club that figures in the story of “Fire,” but with the string section, not the jazz combo, taking center stage during the bacchanal. “It’s the sexiest sound those Met strings will ever make,” Moore said after a rehearsal. “You could see that they were feeling it!”In an interview, Blanchard tipped his hat to an early teacher, the composer Roger Dickerson, who used timbres and modes from American jazz when writing classical works like the New Orleans Concerto. (The pianist, composer and critic Ethan Iverson recently lavished praise on that rarely heard piece, describing its finale as “boogie-woogie gone surreal, the kind of thing Louis Andriessen tried to write over and over again, but better.”)When Blanchard started working with classical musicians, as he has done in his long partnership with Spike Lee as the composer of his soundtracks — Dickerson informed him that he had a unique opportunity, and a responsibility.“‘You have to keep in mind, the library of music for orchestral music has been limited,’” Blanchard recalled his teacher as saying. “‘There needs to be an expansion of it, through jazz — and maybe you’re the person to do that.’ He put that in my mind way back when.”Blanchard’s score for “Champion” synthesizes the varied musical genres he has taken in during his career as a composer and performer.Ike Edeani for The New York TimesBlanchard, who in 2021 became the first Black composer to have his work staged at the Met, has moved opera forward in exactly that way with his latest revisions to “Champion,” Nézet-Séguin, the Met’s music director, said.Even as the conductor has offered small suggestions in rehearsals — like proposing a bit of bowed, marcato playing for the strings instead of pizzicato that could get lost in the Met’s grand auditorium — he has also deferred to Blanchard, who he said has been “much more hands on” about fine-tuning the orchestration.“I think he’s using the orchestra not to amplify his thoughts,” Nézet-Séguin said. “It’s more: How can I use it as a vehicle, the same way I would use a band? It doesn’t replace anything; it becomes its own thing.”Looking up at the stage after a recent run-through of “Champion,” Nézet-Séguin added of Blanchard, with a touch of pride in the musicians: “I’m pretty sure that in his next ventures — whether it’s film music, or whatever it is — he’s going to miss all that.” More