American Stages 2006-2007 season was relatively conservative, reflecting the tastes of its graying audience. But one show at the years end was anything but staid: Casa Blue: the last moments in the life of frida kahlo. This ode to the Mexican artist was distinctly avant-garde: grating, uncompromising, alien and unnatural. Four different actresses played Kahlo at various stages of her life, the movie Frankenstein played on a screen in the background as ghoulish marionettes fell from the ceiling like injured demons, an atmosphere of nightmarish grotesquerie pervaded every scene. Writers Jeremy Childs, Karen Garcia and (American Stage artistic director) Todd Olson offered not a typical biodrama but a visual phantasmagoria, macabre and unpredictable and not only was the audience not offended, but the demand for the play led to an extended run. Maybe that gray audience isnt so conservative after all.