If its true that youve got to suffer if you want to sing the blues, as David Bromberg proposed in one his best tunes, then how to explain the Mill Valley Film Festival? The living is easy in Mill Valley in October, with the gauzy fall sun casting its delicate rays on polite groups of pretty people, smartly (yet casually) clad in Ralph Lauren. Yet the festival overflows with the cream of the season's seriously downbeat films, from wrenching wartime romances (Lust, Caution, one of the opening night choices) to sudden widowhood (Things We Lost in the Fire) to secret U.S. interrogation of suspected terrorists (Rendition). The heavy sledding extends to a trio of tense films revolving around children, whether missing (Gone Baby Gone), relocated (Warchild) or dead (Reservation Road). It may be a paradox, but it's a winning formula, for the fest marks its 30th year with a loyal audience, a stellar rep, financial stability, and a boatload of name filmmakers and actors, including Ang Lee, Laura Linney, Todd Haynes, Jennifer Jason Leigh, Marc Forster, Ben Affleck, and Mira Sorvino. On the other side of the Marin County line, the blues goes down nice and smooth.