How to liberate a blockade in Pirates Tides of Fortune:
First up ye probably want to scout the Haven to make sure that the defending army is not strong. Once that is done ye click in the middle, on the Haven itself, to enter it. Then on the top left hand side ye see the pirate that is blockading. Click on that and ye get a menu of different actions to choose from one of which is Liberate.
Once you click Liberate ye will be taken to the Reinforcements window to select the army ye wanna send.