That blockade of course was an agressive act against Cuba, and it was the correct course of action IMO because a rule that says that no country will be agressive towards any other country is unworkable because there is no power above the countries that could enforce the rule. (The UN is toothless.) If a country's leaders adhere to an unenforceable rule and insist on other countries doing the same, that will on average lead to more death and less prosperity for the world than if each country's leaders rationally pursues their country's interests IMO.
Some courses of action at the country level are more ethical than others, but it is more complicated than you imply it is and any ethical framework has to take into account certain realities.
