How were they used?
Ships and blockades were used to stop trade through the seas. The North wanted to starve the cities and then attack them when they were weak and hungry. This is a very well known tactic in warfare and is still used to day.
How many ports were blockaded during the Civil War?
The Union Blockade was proclaimed by President Abraham Lincoln in April 1861, and required the monitoring of 3,500 miles of Atlantic and Gulf coastline, including 12 major ports, notably New Orleans and Mobile. Those blockade runners fast enough to evade the Union Navy could only carry a small fraction of the supplies needed. They were operated largely by British citizens, making use of neutral ports such as Havana, Nassau and Bermuda. The Union commissioned around 500 ships, which destroyed or captured about 1,500 blockade runners over the course of the war.
Were they effective?
The blockades were very effective they starved out cities and towns. Now some ships did get through but these are only small fast ships that run with the wind the, bigger ships were stopped.