No one in Saks Fifth Avenue yesterday recognized the blond woman in a blue suit with turquoise earrings as Tricia Nixon Cox — except for historian Barry Landau, author of “The President’s Table.” Cox told Landau she’d recently visited the White House, where Michelle Obama introduced her to daughters Sasha and Malia, and told Cox that one of the first couple’s favorite photographs, which hangs near Michelle’s East Wing office, shows Tricia walking down the aisle on the arm of her father, then-President Richard Nixon, in 1971 to swap vows with Ed Cox, who is now head of the New York Republican Party. Tricia’s and Ed’s son, Christopher Cox, who just got engaged to Gristedes heiress Andrea Catsimatidis, is running for Congress in Suffolk County. President Obama — who recently laid a wreath on Nixon’s grave at his library in Yorba Linda, Calif. — added that he was “looking forward” to his and Michelle’s own daughters’ weddings, “but no time soon.”