During the Cuban Missile Crisis, the Kennedy Administration was careful to label its naval action to stop further deliveries by Soviet ships a "quarantine." The reason for this was that a "blockade" is an act of war under international law.
I don't know all the legal and diplomatic consequences that flow from calling the Iran operation a blockade instead of a quarantine. As an Admiralty teacher, all I can say is that acts of war trigger mutual exceptions or force majeure is most maritime contracts.