There may well be something in what you say....I found this: "In order to be binding a blockade must be effective".
http://definitions.uslegal.com/b/blockade/
And this: "Blockades were first defined in
international law at the
Paris Declaration Respecting Maritime Law 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"
http://en.wikipedia.org/wiki/Blockade
The quote above actually goes some way into explaining why 'paper blockade'.