Howard University’s Afro Blue Reunion Choir rehearsed for 18 hours over the span of a few days before joining voices with singer Bobby McFerrin at the Warner Theatre on Saturday night. And McFerrin? He winged it as only he can.

Actually, that’s not entirely true. McFerrin knew his role thoroughly, contributing to a series of complex and challenging vocal orchestrations drawn from his recent CD, “VOCAbuLarieS” — tunes that spanned a world of cultural influences. But during this a cappella performance, he sang with such effortlessness and invention that he often appeared to be improvising even when he wasn’t.

His voice was constantly shape-shifting: now a flute, now a bass, now a saxophone, now a — jeez, what was that, a didgeridoo? He didn’t sing Paul McCartney’s “Blackbird” so much as uncage it, and when the creature took wing, the wizardly Grammy-winner managed to create a fluttering Doppler effect in its wake.

Then, too, there were moments — many moments — when McFerrin simply tossed out the script. Early on, he was so moved by the choir’s rich and stirring harmonies that the sounds stopped him in his tracks — literally. The next thing you knew, he was channeling a street-corner serenader from the doo-wop era, lavishing praise on the choir in a Frankie Lymon-like croon.

Under the direction of McFerrin collaborator Roger Treece for this D.C. Jazz Festival performance, the choir deserved all the applause it received for navigating intricately devised vocal arrangements and contrapuntal passages and for fully charging the evening’s most inspirational song, “Messages.” Some two dozen singers strong, the group often generated syncopated or undulating waves of harmonies, then layered and contrasted them for dramatic effect.

The longer McFerrin sang, the more interactive the performance became. He invited members of the choir and audience to take turns as his duet partner — a baptism of fire for singers unaccustomed to the spotlight. The volunteers not only managed to rise to the occasion, they triumphed, delighting McFerrin and the audience with their nerve and talent.

With the choir behind him, stretching from wing to wing, McFerrin essentially performed in the round. One of the evening’s consistent pleasures was watching the young singers drop their jaws or smile in brilliant unison when McFerrin was engaged in some high-wire daring, either alone or with colleague David Worm. A singer and “vocal percussionist,” Worm matched wits with McFerrin during a freely improvised, bop-to-Brazil-and-beyond interlude.

McFerrin, of course, has never met an audience that he couldn’t turn into a choir. On this occasion, he was abetted by an unusually tuneful crowd that included at least one terrific soprano. Her voice soared to heavenly heights during “Ave Maria,” while McFerrin precisely underpinned the Bach framework. In the end, when virtually everyone in the house was standing and applauding, including all the performers, McFerrin seemed every bit as happy with the concert as his fans, which is saying a lot.

Joyce is a freelance writer.