The Advent portion began with a set based on what are known as the "O antiphons," music that praises various divine attributes, written explicitly for each of the days prior to Christmas. "Veni, veni, Emanuel" was sung in darkness, perhaps in a nod to the winter solstice.
The set also included "O rex virtutum," a work based on the antiphon written for Dec. 22, the date of the concert. Blue Heron's signature purity of intonation and clarity of line were both in evidence, though as the number of singers expanded, balances did not always reflect the precision this group can achieve at its best.
A set of charmingly direct medieval carols followed, with Blue Heron director Scott Metcalfe picking up a harp for the occasion, and later the vielle. With a text in Middle English, "Ther is no rose of swych vertu" from the Trinity Roll manuscript was a standout, with Daniela Tosic's vocal lines elegantly cushioned by Mark Sprinkle and Paul Guttry.
Throughout the program, expressive immediacy took due precedence over imagined notions of authenticity, so the Middle English carols were rendered with modern pronunciation, and the anti-Jewish vitriol in works such as Nicolas Grenon's "Nova vobis gaudia refero" was deftly airbrushed out, in this case by Larry Rosenwald.
Much of the eclectic New Year's material departed from the sacred realm in favor of the secular pursuits of courtly love. Levels of polish were less uniform in this set, but it came to a rewarding culmination in Du Fay's "Je requier a tous amoureux," distinguished by the refined tone and eloquent phrasing of mezzo-soprano Pamela Dellal.
A second Christmas-themed set was capped by the remarkable Cypriot motet "Hodi puer nascitur," which suspends a fervent Christian mysticism in a rich web of polyphony. Here again, the choir sang with the accuracy and immediacy required for this music to address a modern listener with an intensity undimmed by the centuries.
Jeremy Eichler can be reached at jeichler@globe.com.
