No, they are not blocking legally registered tankers from other countries. The handful of boarded ships were boarded because they were flying false flags, which is illegal and opens them up to being seized regardless of the embargo.

So if Russia puts a Russian flag on a tanker and sails it to Cuba, do you suppose that the USGC will allow it to land?

Oh, wait. Those ships are all sanctioned so would be seized. Interesting conundrum.