In the 1960s, “minimalist” composers Terry Riley and Steve Reich broke from mainstream classical music with compositions that shunned complex formalism and introduced improvisational elements. Baltimore’s “early music” chamber ensemble Harmonious Blacksmith ups the ante by performing this boundary-pushing modern music on recorders, harpsichords and other Renaissance and Baroque instruments. This afternoon they perform Riley’s “In C,” 53 short phrases repeated in a mesmerizing pattern, and Reich’s “Piano Phase,” in which two harpsichordists perform identical pieces at different speeds, creating surprising melodies and contrasts.