What does blockading of ports do ?
Helps with sieges (by cutting sea supplies) & lowers the income of the province to about half of normal. The latter can economically cripple an opponent with many coastal provinces. A much prefered tactic for any naval power like England versus France. Remember though that before naval tech 19 you do not have access to blockading technology. To do the blockade once your at naval 19+ and at war with an opponent, take a sufficently large fleet and place it in the sea area outside any enemy harbour. The province capital will have padlocks in the info screen to show it is now under blockade.
