Elected officials lining up with labor unions to block the opening of Walmart within the five boroughs “should have trouble looking at themselves in a mirror,” Mayor Bloomberg declared yesterday. The mayor’s comments came five days after it was announced that the Related Cos. had signed a deal to open a Shop Rite supermarket in the East New York section of Brooklyn at a site that Walmart had been considering for months. “[Unions] try to pressure local officials,” the mayor said. “The fact that some elected officials caved to that pressure does not do [them] any good with the public, and also, they should have trouble looking at themselves in the mirror.” Bloomberg pointed out that thousands of New Yorkers already shop at Walmart, except they do so in neighboring suburbs, where the retailing giant has been attracting New Yorkers and the revenues they generate for years. City Council Speaker Christine Quinn, another Walmart critic, did not comment.