The traditional IL on naval blockades is:
1. It is an act of war.
2. It is declared by one of the warring parties.
3. It is lawful as long as it can be enforced.
4. It can be enforced by whatever means are ncessary - traditionally, that was seizing a blockade runner, imprisoning its crew, and seizing its its cargo, or blowing it out of the water.

Cheers

JohnT