2. Lincoln deliberately chose to proclaim a blockade despite its legal implications
Under international law, a nation does not blockade its own ports. Instead, it closes them, as the British did the port of Boston in 1774. Proclaiming a blockade of the southern ports implied that they were recognized as a belligerent nation, rather than states embroiled in an insurrection against the legitimate government. Lincoln’s proclamation was called a blunder by politicians at the time and by some historians since. But it was not a blunder, Lincoln chose the word after considerable deliberation. It provided the United States with a legal right under international law which closing the ports would not. The right was useful for prosecuting the war.
Ships patrolling outside of a port which was legally closed did not have the right to stop ships of neutral nations and inspect their cargoes for contraband. Ships patrolling outside of a blockaded port did have that right, as well as the right to seize ships carrying contraband and send them into another port for adjudication under maritime law. Lincoln’s use of the word blockade in his proclamation gave the ships of the US Navy the power to enforce it against all ships bound for the United States on the open sea. It could thus stop ships leaving Caribbean ports and inspect their cargoes, as well as those found on the American coast.