Blue is right.
It's the appropriate color to represent a husband and wife who have fallen out of love and have nothing but contempt for one another.
When we first meet Cindy (Michelle Williams) and Dean (Ryan Gosling) in their sad, small kitchen, she's doing her best to get herself and their daughter fed and ready for the day. He's trying to be a jokester and is doing nothing but hampering her efforts. There's little love or affection on display here. There may not be any left in the marriage.
We see a bit more of the couple's painful present-day life together before we're taken back five years or so to watch them meet for the first time. He's instantly smitten. She's a serious student, from modest means, with a sometime-boyfriend. Dean's persistence wins her over. Even as we watch their tentative, awkward courtship and see them falling for each other, we see signs of trouble.
The action takes us back and forth in time between the early days of their relationship and, a few years hence, when the two are completely disappointed in themselves, their choices and each other. (It's never tough to know where in time we are because the characters change so much, emotionally and physically, in the course of a couple of years.) Loser Dean can't live up to the selflessness he first showed Cindy when they met. And, Cindy was too smart -- or should've been -- to have married for convenience someone she barely knew.
Yet, here they are in abject misery. Dean tries what little he can (a "sex motel" for a night, for instance) to reignite their spark. But, you get the sense that it's coming a bit too late.
Cindy is done. The way Williams plays her is a marvel. Her perpetually slumped shoulders convey a woman much older and utterly without hope.
As dark and dismal a tale as you're likely to see on screen this year, Blue Valentine is a masterpiece. It's one of the year's best, and Williams and Gosling are wondrous to behold in all their pain. See it!