The Sing-Off is, like all musical reality shows, steadfastly and proudly middle-of-the-road. But juxtapose its concept with performers whose forte is elswhere, and the middle of the road becomes a challenge in spite of itself—-something D.C.’s own Afro Blue discovered on last night’s installment of the show, in which the Howard University ensemble passed round 2 of the competiton.

“Afro Blue does not do pop songs at all,” member Danielle Withers said in an artist-profile video, explaining how the group had to work to fit its chosen song, Estelle’s “American Boy,” to its sound. No easy task, but Afro Blue did an impressive job, reharmonizing the tune with dense but effortless jazz chords and a vocal rhythm reminiscent of the group’s work this summer with Bobby McFerrin. Christie Dashiell opened the tune with a shrewd piece of scat, and Reggie Bowen provided superlative bass support for Withers’ lead. It was a performance that earned a “WHOOP!” from judge Shawn Stockman (who seems particularly high on the group). Sara Bareilles (who seems particularly high on everyone) let loose a “damn” and a “hell,” explaining that excitement brings out her “potty mouth,” and Ben Folds praised Afro Blue for removing the elitism from jazz and “driving the point home.”

Radio hits, however, were only half of the assignment this week; the contestants also had to arrange and perform a ’60s standard. The choir went with Marvin Gaye‘s “I Heard It Through the Grapevine.” However, in this case they opted for a fairly complicated arrangement that got even more so near the end, when it undertook a hefty modulation and an ornate coda with backing vocalists singing, “Everybody was talkin’ about you” in offset harmony. Impressive work, but, as Folds said, “You overthought it a bit too much.” (Not an uncommon occurrence in marrying cerebral jazz structures to pop music’s to-the-gut approach, though Motown tunes were never as simple as they seemed.)

Notably, though, Afro Blue is still avoiding the kind of showy, explosive crescendoes that are the crowd-pleasers on this show. They’re showy in a chopsy sense, not a dynamic one, and it seems to sit well with the judges. Hence, Afro Blue made it through again this week, while L.A.’s Kinfolk 9, who were bland in their first appearance and overwrought in their second, were sent packing.

One final note: Afro Blue is doing quite well, and that can be attributed largely to their less conventional approach to a cappella performance. But they’ve got serious competition in that category from a Denver-based group called Urban Method, whose nucleus is rapper Myke Charles and uses two beatboxers as well as standard harmony parts to create a cappella hip-hop. (This week Urban Method did strong renditions of Black Eyed Peas‘ “Just Can’t Get Enough” and Sly and the Family Stone‘s “Dance to the Music.”) These are two groups that have carved out distinctive niches among the performers, and theirs will be a competition to watch.