Under international law, does Eritrea have a right to blockade or impose heavy tariffs on goods moving to Ethiopia?
Many countries who are landlocked do this:
- Use navigable rivers to get to the sea if the nations through which the river goes through are friendly nations.
For example: Austria uses the Danube or Rhine for sea access and the European Union’s Schengen area further encourages Austria to do so
- Build strong diplomatic relations with a neighboring coastal country.
For example: Ethiopia uses their friend Djibouti’s sea access. Another example is Switzerland or Austria using Germany’s sea access.
- Militarily gain access.
Examples are rare in the modern world but one such thing happened when Russia invaded Crimea to gain more access to the Black Sea through the port of Sevastopol in Crimea. Although they have a huge coastline It is frozen for most of the year. So you can say Russia is technically ‘Icelocked’ Although Russia has the warm water port of Novorosyssk on the Black Sea Russia also needs a secondary port.
A blockade is an act of war:
Blockade, an act of war by which a belligerent prevents access to or departure from a defined part of the enemy’s coasts.
Blockades are regulated by international customary law and by international treaty law. A blockade must be declared in advance by notification of all neutral powers, and it must be applied impartially against ships of all states. (Encyclopaedia Britannica)
What you mean is something else: Is Eritrea allowed to not allow goods be transported to or from Ethiopia? Can Eritrea close its border with Ethiopia?
Of course! Any country can close its borders as it wants. It's part of it's sovereignty. In particular, the border between the two mentioned countries had been closed for a long time. (see EastAFRO.com)
This doesn't exclude the existence of internationally binding agreements, for example, the Montreux Convention Regarding the Regime of the Straits which obliges Turkey to let civilian ships pass through the Dardanelles and Bosporus in peacetime.
Generally speaking, yes. It would not be a blockade, which means warships in interational waters stopping neutral traffic. Eritrea can decide if they want to trade with Ethiopia or not. Ethiopia has five other countries left to trade with, from Sudan to Djibouti.
An embargo is a refusal by one country to sell to another country.
A boycott is a refusal by one country to buy from another country.
A blockade is one country trying to stop third parties from trading with another country through international waters and airspace. It is usually an act of war.
Sanctions are usually a combination of embargo and boycott actions, but not blockades.
Some countries try to pressure third countries to join their sanctions, effectively saying "if you are not for me, you are against me -- take your pick." That is only possible for powerful countries like the United States.