For the time being, the US did not declare any blockade. It is simply seizing vessels that are part of the "shadow fleet". The particular oil tanker that was seized recently was flying under a Guyanese flag. Guyana, by the way, is Venezuela's neighbor, and about a year ago Maduro was thinking of invading it. Guess what, Guyana said they have no idea why that tanker is flying their flag, so technically speaking the ship was flying a false flag, therefore under the UN Convention of the Law of the Seas (UNCLOS) the vessel could be boarded, inspected and seized.