It is not a blockade. Idiots. So if there is no blockade, it is not "amid US blockade".
A blockade is a specific legal thing, an act of war. Where you stop all ships using military means. The US is using legal means to make it very expensive for countries like Mexico to send oil, which is not a blockade.
So it is not surprising that the US will not stop these Russian ships.
The defining characteristic of Trump's foreign policy is that keeping promises, integrity, consistency, truthfulness, morality, and legality have no benefits for the US. Only raw power matters, and dollars, according to Trump. Trump has done his level best to destroy US soft power.
Which is pretty much how Iran itself works. E.g. 2026 Iran massacres. So if you don't cheer for the US, fair enough, but please don't cheer for terrorist Iran.