> Seizing ships isn’t a blockade.
> Turning away ships isn’t a blockade.
If the vessels were legally flagged, both of these are indeed actions of a blockade!
You're just ignoring the fact that the ships the US seized were flying false flags and are subject to seizure regardless of the embargo.
And in the case of the Ocean Mariner, the ship wasn't forcibly turned away by the coast guard. They changed course to the Dominican Republic (which was its purported destination anyway...) on their own volition when the realized they were being tracked. The could have continued to Cuba if they wanted to, but that would trigger retaliatory tariffs.
> The Cuban Missile Crisis doesn’t even meet your standard.
Yes it was a blockade! The US military deployed its forces with orders to seize Soviet ships bound for Cuba (though they turned away before any ships were actually boarded).