BIDEN’S BLUE FRIDAY: Joe Biden was all smiles chatting with Centre Street shoppers Friday afternoon on Nantucket. Looking very “un-vp” in a navy bomber jacket, khakis and a baseball cap, he was still up for photo ops. When Terry Ventre handed off a camera for a snap of the vice president with her teen daughter Ruby Zolot to Biden’s granddaughter, Secret Service agents intervened. But Biden’s kin insisted on taking the shot and while doing so, the mother led the passersby who had gathered in singing “Happy Birthday” to President Obama’s wingman, who turned 70 days before. Noticing how the presumably blue-stater teenager had flushed red, Biden kissed her twice on the forehead saying, “This is what mothers are supposed to do — embarrass their daughters.”