A naval blockade would be a strategic move to interdict North Korean missiles in addition to armament the U.S. has on the ground. This would be its primary objective.
Remember the bulk freighter named ‘Jie Shun’ that was flying Cambodian colors but had sailed from North Korea with a North Korean crew and an unknown cargo covered by heavy tarps?....after taking over the freighter they found, under bins of iron ore, a cache of 30,000+ rocket-propelled grenades. It was the largest seizure of ammunition in the history of sanctions against N.korea.
N.Korea has had years to learn how to get around sanctions. And that includes ‘shipping armament’....as you say a blockade would stop this and further strangle Fatboys cash flow for his Nuclear ambitions....which he’s very close now in obtaining.