Which Law or treaty did the United States sign before the Civil War that stated positively that blockading ones on ports was illegal. By Declaring a naval blockade of Confederate ports, Lincoln could legally stop and search neutral (British, French, or other European) vessels for contraband in international waters. The declared blockade allowed the Federal navy to search cargos of neutral ships and seize cargos such as weapons, gun powder and the ships carrying them. Merely “closing” a port does not bestow that authority.
As to your question, no treaty that I am aware of prohibited blockades of one's own ports. Nor, to the best of my knowledge, was there any "law" that the US signed to that effect. But that does not exhaust your inquiry. As you may recall, prior to the War of Northern Aggression, Brittania ruled the waves. Just as Great Britain gave us the system of common law (made by judges ruling on actual cases and controversies). Likewise, the British admiralty had an interest in having naval controversies governed by a common law of the seas.
Since there was no constitutional authority to blockade what Dishonest Abe claimed were American ports, his declaration of blockade was illegal (see the 10th Amendment) and conferred absolutely NO AUTHORITY on anyone to stop or search any ships of any nation whatsoever.
There was an instructive incident involving a British mail ship plying the waters of the Carribean before heading home to England. On board HMS Trent were two important Confederate diplomats, Slidell and Mason, who had been appointed by Jefferson Davis to be ambassadors to Great Britain and France. They carried with them diplomatic pouches containing confidential documents. The US Navy under a Captain Wilkes (?) stopped the Trent, searched her, found Slidell and Mason and their diplomatic pouches, placed them under arrest and brought them to the brig at the Boston Navy Yard.
The diplomatic community of many nations exploded in response to such an unheard of interference with diplomats. The infant New York Slimes editorialized in admiration of Captain Wilkes. Lincoln called a cabinet meeting and asked the cabinet members present to vote on whether to free the diplomats and their pouches. All 8 cabinet members voted to keep them in jail. Lincoln disagreed and announced that by a vote of one in favor of their freedom and eight against they would be freed.
Lincoln also had no authority to close any port claimed to be American.