With “The Place Beyond the Pines” arriving in theaters with a bang last month, now may be a good time to remember “Blue Valentine,” Derek Cianfrance’s debut film of 2010 that critics called “astonishing” and audiences confirmed as such. It was and remains so on second viewing, this story of the unraveling of a marriage, for reasons all too familiar yet still unexplainable. What happens to people in a couple, how does a spouse go from