WASHINGTON — In a rare diplomatic rebuke, the United States has blocked Iran’s controversial pick for envoy to the United Nations, a move that could stir fresh animosity at a time when Washington and Tehran have been seeking a thaw in relations. The Obama administration said Friday that the U.S. had informed Iran it would not grant a visa to Hamid Aboutalebi, a member of the group responsible for the 1979 takeover of the U.S.