The second session of the 80th season of the Stern Grove Festival — the yearly 10-week summer concert series — promises to be a gem. Bluhm, coming off her performance at the Monterey International Pop Festival 50th anniversary celebration, brings her soulful voice and her band’s infectious country rock and folk sound, while Fantastic Negrito brings his mix of blues and black roots music, which recently won him the Grammy Award for best contemporary blues album. The free concert takes place in the unique Stern Grove amphitheater, where optimal acoustics are isolated in the grove’s lush natural surroundings.