• So taking the first one on that list:

Moments before the seizure, the tanker was flying the Guyanese flag, despite not being registered in that country.

It is legal to seize ships flying a false flag. So that seizure was not an illegal blockade by the US.

• Second one is a Venezuelan one, which fair enough the US is acting illegally in all kinds of ways with Venezuela, nobody serious disputes that. But Venezuela is a special case.

• Third one was a Panama flagged ship, and "Permission to board was obtained from Panama". So boarding that was legal.

Etc etc I assume. My point being, it is not a complete blockade - the US is going out of their way to specify legality (excluding Venezuela...). The US is not going to illegally board the Russian ships heading for Cuba.