http://en.wikipedia.org/wiki/Blockade#Governing_laws

Soyez particulièrement attentif à la dernière phrase, Pegasus :

Whether or not a blockade was seen as lawful depended on the national laws of the nations whose trade was influenced by the blockade. The Brazilian blockade of Río de la Plata in 1826, for instance, was considered lawful according to British law, but unlawful according to French and American law. The latter two countries announced they would actively defend their ships against Brazilian blockaders, while Britain was forced to steer for a peaceful solution between Brazil and Argentina.[5]

Blockades were first defined in international law at the Congress of Paris in 1856. One of the agreed rules was that a blockade had to be effective in order to be lawful. This banned so-called « paper » blockades — blockades that were declared to the blockaded nation, but were not actively enforced, allowing the blockading party to seize the cargo of neutral states trading with blockaded harbors.[6] At the Declaration of London in 1909 another attempt was made to further protect the rights of neutral traders.[7] The treaty was only ratified by a few nations, preventing any application of the agreements. Parts of it were, however, applied during blockades in World War I.

Since 1945, the UN Security Council determines the legal status of blockades and by article 42 of the UN Charter, the Council can also apply blockades.[8]