The Defense Department has estimated that the U.S. naval blockade has cost Iran about $4.8 billion in oil revenue, a Pentagon official familiar with the assessment told The Hill late Friday.  The blockade, which was instituted more than two weeks ago, is one of the actions President Trump has used to inflict pressure on Iran as negotiations with Tehran appear to be at a stalemate.  “The United States’ blockade in the Strait of Hormuz is operating with full force and delivering the decisive impact we intended. We are inflicting a devastating blow to the Iranian regime’s ability to fund terrorism and regional destabilization. Our Armed Forces in the region will continue to maintain this unrelenting pressure,” acting Pentagon press secretary Joel Valdez  said  in a statement on Friday.  Since the blockade went into effect on April 13, the U.S. forces in the Middle East have forced 45 commercial vessels to turn around or return to port, U.S. Central Command (Centcom), which oversees U.S.…