The International Committee of the Red Cross has been quoted - even by the BBC - as saying that the blockade is illegal. However, that is not true - they said that the closure of Gaza was illegal where Israel limited the types of goods allowed in before last summer. The word "blockade" in a legal sense refers specifically to the naval blockade by Israel of an enemy territory. The Red Cross was very careful not to use the word "blockade."
Amnesty and a host of other NGOs were not as careful, as they - without citing any evidence or legal reasoning - referred to the blockade as "illegal" in a report issued last year.
The UNHRC, in its laughable flotilla report, actually tried to find legal reasonings why the blockade is illegal:
In evaluating the evidence submitted to the Mission, including by OCHA oPt, confirming the severe humanitarian situation in Gaza, the destruction of the economy and the prevention of reconstruction (as detailed above), the Mission is satisfied that the blockade was inflicting disproportionate damage upon the civilian population in the Gaza strip and that as such the interception could not be justified and therefore has to be considered illegal.
Given that Gaza has no ports to import goods, it is absurd to say that the naval blockade is disproportionately punishing Gazans!
Wikipedia summarizes the governing laws of a blockade:
According to the San Remo Manual on International Law Applicable to Armed Conflicts at Sea, 12 June 1994, a blockade is a legal method of warfare at sea, but is governed by rules. The blockading nation must publish a list of contraband. The manual describes what can never be contraband. Outside this list, the blockading nation is free to select anything as contraband. The blockading nation typically establish a blockaded area of water, but any ship can be inspected as soon as it is established that it is attempting to break the blockade. This inspection can occur inside the blockaded area or in international waters, but never inside the territorial waters of a neutral nation. A neutral ship must obey a request to stop for inspection from the blockading nation. If the situation so demands, the blockading nation can request that the ship divert to a known place or harbour for inspection. If the ship does not stop, then the ship is subject to capture. If people aboard the ship are resisting capture, they can be attacked. It is still not allowed to sink the ship, unless provision is made for rescueing the crew. Leaving the crew in liferafts / lifeboats does not constitute rescue. If a neutral ship is captured, any member of the crew, resisting capture can be treated as prisoners-of-war, while the remainder of the crew should be released. A neutral nation may choose to send a convoy accompanied by warships. The warship can provide guarantees that the convoy does not contain contraband. in which case, the blockading nation does not have any right of inspection.
Israel has fastidiously adhered to all of these requirements.
It is nice to see that the UN has the ability to tell the truth once in a while. It remains to be seen if this report will ever see the light of day.