Maybe the MPAA had a plan when they gave “Blue Valentine” an NC-17. To date, the only such-rated film to land an Oscar nomination is “Henry and June” (for cinematography), but if “Blue Valentine” were to earn recognition in top categories — screenplay, performances and picture all seem plausible, given the rapturous reception at the Sundance, Cannes and Toronto fests — surely the world would have to reconsider the stigma that surrounds the adults-only rating. Right?One can hope, though it didn’t work out so well for the X. Even after “Midnight Cowboy” took 1969′s top prize (and “A Clockwork Orange” was nominated two years later), mainstream theaters continued to shun X-rated films, and many newspapers still refused to run their ads — two problems that still face NC-17 offerings today, making it that much harder for “Blue Valentine” to compete for awards consideration. Never explicit but certainly concerned with grown-up issues — namely, when to call it quits on marriage — “Blue Valentine” serves as a 21st-century spin on “Two for the Road,” alternating between scenes of early courtship and the rocky final days of a relationship. Starring former Oscar nominees Ryan Gosling and Michelle Williams in career-best performances, the film also signals a major new talent in director Derek Cianfrance, leaving pundits to wonder: How will it rate with the Academy?