Sonya Yoncheva doesn’t fill out the long lines of “Norma” at the Met, while Angel Blue is a warm, sincere Violetta in “La Traviata.”“An irresistible force drags me here,” a character says of the man she loves in Bellini’s “Norma,” which was revived at the Metropolitan Opera on Tuesday with the soprano Sonya Yoncheva in the title role. “The breeze echoes with his dear voice.”Every opera, of course, wants the voices in it to be irresistible forces, echoing in our minds; that is the point of the art form. But in the bel canto works of the early 19th century — of which “Norma,” from 1831, is a lasting masterpiece — vocal quality is more than a want. It’s a need.Particularly in the monumental title role. Norma is a descendant of Medea, a character who opened the Met’s season in Luigi Cherubini’s 1797 opera. Both are women wronged by their lovers and contemplating the murder of their children; both are figures of immense, mystical stature. And in both works, the drama lies in the breaking down of their authority: the revelation of an archetype, a myth, a goddess who is also a woman.In bel canto works like “Norma,” the protagonist’s grandeur, the heights from which she falls, are established by the soprano’s vocal technique, by the long, confident musical lines she spins. Bellini’s orchestra is subtle and sensitive, but austere enough that this opera’s stakes are purely vocal. If the score isn’t sung beautifully, it’s not simply bad — it’s almost nonexistent, which is the case in the Met’s drab revival.Over the past decade, Yoncheva has risen from a series of last-minute fill-ins to solo recitals on the Met’s stage and starring roles in new productions, including Umberto Giordano’s 1898 potboiler “Fedora” this past New Year’s Eve. But even for an established leading lady, Norma, which Yoncheva first sang in London seven years ago, is a daring proposition.As this druid high priestess, caught in a forbidden love triangle with a Roman soldier and a fellow priestess, Yoncheva can be forceful in declamation — the singing that’s more like speechifying. And she’s long been able to convey the sense of a character thinking as she sings.But crucial to this score, as to all bel canto, are the seemingly endless, time-defying lines that, on the revival’s opening night, she struggled to sustain, with an unsettled vibrato and big, gulping breaths breaking up core arias like “Casta diva.” Without powerful, poised, flexible singing — “beauty of tone and correct emission,” as Lilli Lehmann, a great Norma, put it — we feel none of the necessary awe for the character. So her fall from grace and the opera she dominates both lose their meaning. While Yoncheva doesn’t betray Bellini’s score, she doesn’t fill its sails, either, and the boat stagnates.The result is a kind of pencil sketch of “Norma” — not imprecise, but colorless. Yoncheva has coloratura agility, retained from her early days as a Baroque specialist, and isolated high notes pop out clearly. But when those notes are the climaxes of arching lines, they’re thin. She is spirited and scrupulous, and her voice is not ugly, but it’s inadequate for this music.The soprano Sonya Yoncheva comes to the Met’s latest revival of “Norma” after rising from a series of last-minute fill-ins to solo recitals on the Met’s stage and star turns in new productions.Marty Sohl/Metropolitan OperaShe neither loses control nor takes real command. And it’s not just strength you can’t convey if you’re not vocally in command as Norma; it’s weakness, too. Yoncheva spends much of the time blandly moping around, small-scale on this soaring canvas.With Maurizio Benini conducting briskly on Tuesday, the rest of the cast, too, lacked the suggestion of the epic. The wayward Roman warrior Pollione is the second big part in a much-anticipated Met season for the acclaimed tenor Michael Spyres, and the second disappointment. There’s a tarnished-bronze, baritonal nobility to Spyres’s voice, but strain in reaching the high register, and a kind of fogged wooliness just below.As Adalgisa, who unwittingly becomes Norma’s romantic rival, the mezzo-soprano Ekaterina Gubanova makes the warmest outpourings of sound onstage, and her classic duets with Norma are neatly done. The bass-baritone Christian Van Horn presses out muscular tone as Oroveso, Norma’s father. In the small role of Clotilde, Norma’s aide, the soprano Brittany Olivia Logan sings with creamy urgency.The sighing “ba-dum, ba-dum” motif in the prelude to Act II anticipates Verdi’s “La Traviata,” which premiered just 22 years after “Norma” and mines that same motif for the same pathos. But by midcentury, operatic orchestral music had increased in density and complexity, and had begun to develop into a character in its own right. And “Traviata,” which returned to the Met on Saturday afternoon, is a far more naturalistic melodrama than the carefully antique, stylized “Norma.”So, unlike “Norma,” “La Traviata” makes its impact — it breaks your heart — pretty much no matter what. (By Giordano and Puccini’s time, 40 or 50 years later, operas were even more indestructible.) Which is not to say that “Traviata” can’t be derailed by its star. Or that it doesn’t bloom with an excellent one, like the soprano Angel Blue, who took on the role of Violetta at the Met on Saturday.The tricky curlicues and fast lines of the first act are sometimes not quite secure for her, and in “Sempre libera,” which brings down the Act I curtain, she exudes vague contentedness rather than bigger, riskier feelings. But even in those opening scenes, she is a warm presence — warm vocally, too, but with a quickly vibrating shimmer to her tone that keeps the sound buoyant and refreshing.There is no cynicism or hardness to her conception of the role, just the woundedness of a quick-smiling woman who has trusted too easily. Blue’s Violetta is always human-size, even in full, rich cry in her confrontation with Germont, the bourgeois father seeking to tear his son away from a liaison that threatens the family.She shows restraint in the third act, not milking the music for extra emotion. Her “Addio del passato” was brisk and bleak; her “Gran dio,” angry rather than pleading. The irrepressible Nadine Sierra and the scorched-earth Ermonelo Jaho offered accomplished Violettas at the Met earlier this season, but the sweet, sincere Blue — who lets the tragedy patiently unfold — may be my favorite.The tenor Dmytro Popov is an earnest, ringing Alfredo; as his father, the disapproving Germont, the baritone Artur Rucinski sometimes forces his seductive tone. In tiny parts, Megan Marino is a sprightly Flora, and, over 600 performances into his Met career, Dwayne Croft (here Baron Douphol) still brings a hearty voice and dramatic investment every time he steps onstage.Michael Mayer’s vulgar production drags down the opera. In the first act, Alfredo warns Violetta, “The way you’re living will kill you,” which makes no sense if, as here, the opening scene has all the demimonde danger of a Hamptons garden party. And, in this period setting, the visibly contemporary labels on the bottles of bubbly come across as yet more lazy summer-stock falsity in a staging full of it.But the show is surprisingly bearable with Blue’s tender honesty at its center. More