Iran did not set up a blockade in the Strait. It set up a toll booth, with a "friendly warning" that the Southern passage through the strait, which is in international waters, might have mines. The Northern passage comes through Iranian territorial waters and they're requiring payment.
The US did set up a blockade against Iran. Per international law, blockade is a form of war consisting of stopping or disabling all ships coming to or from the blockaded nation's ports. It specifically excludes preference to ships of select nations, so a declaration of blockade implies intention and ability to completely cease shipment through the blockaded ports, regardless of who it's from and who it's for.
In practical terms:
- US and Israeli ships will be attacked if they attempt to transit the strait.
- Neutral ships are being requested a toll by Iran (generally not legal).
- Trump has claimed he'll sink neutral ships which pay such a toll (war crime, should be refused as illegal orders).
- As well as ships coming to/from Iran (legal under the laws of war).