The small Boston-based choir Blue Heron specializes in innovative performances and recordings of Renaissance and medieval vocal music but has a broad repertory stretching from plainchant to newly composed works. The group studies historical performances and original source materials closely and often performs rarely heard music, notably the 16th century English Peterhouse Partbooks. Nevertheless, its themed programs offer considerable expressive intensity and are not motivated strictly by historical performance practice. The group has a substantial recording catalog on its own...