They could manage a blockade without bringing surface ships into missile range. Just have a network of patrolling ships with helicopters, use radar to pick up inbound merchants while they're still far away, and politely inform them that going to California is not permitted at this time.
Helicopters generally dont carry radar for one thing; they generally dont have the payload capacity, or the altitude envelope.
And they have limited speed and endurance in any sort of sea search role.
For another, radar can be detected at several times the range at which it will help you detect anything.
Its a bit like shining a flashlight on a dark night; people can see you from much farther away than you can see them. That's why emissions control is an established naval strategy.
The Japanese can operate UAVs and maritime patrol aircraft from Hawaii as search, but a blockade requires actual surface ships in the water for search and seizure purposes.
Because its not the Collapse anymore, and shooting flagged civilian ships of foreign powers carries consequences.
With a decent network of spies, they could also catch outgoing merchants. If you have a man at the docks who notes down the names and descriptions of merchant ships, then you can seize the merchant ships after they leave and impound their cargo. This would seriously discourage people who want to trade with California.
Lemme try to talk about the military picture as I see it, and why its a prohibitive strategic problem given the assets on hand.
Cali's arsenal is derived from early 21st century US military stock, so I'll be using known figures.
The current longrange antishipping weapon of the US Navy air arm is the AGM-158C LRASM subsonic stealth cruise missile, which started development in 2009 and entered full service in 2018. It weighs roughly two thousand pounds, carries a thousand pound warhead, has a passive IR seeker, can home on hostile RF emissions, has an estimated range of >500km, and can be carried by the F-35, the F/A-18, and the P-8 Poseidon long range maritime patrol aircraft.
We know the NCR operates upgraded F-35s by WoG.
Publicly available information says that the F-35A has a combat radius of ~1200km on internal fuel alone; no external fuel tanks, no tanker support.
Range with external fuel tanks is unknown or upgraded engines.
The F/A-18E/F Super Hornet has a combat radius of 1100km while carrying 2x Sidewinders, 2x AMRAAMs, 2x 2000-pound class bombs and a single centerline drop tank(480 gallon, I believe).
That goes up to 1300km if they use conformal fuel tanks.
The P-8 Poseidon is basically a militarized 737-800ERX with surface search radar.
It has a combat radius of 2200km, time on station of 4 hours, 5x internal weapon bays, and 6x external hard points for weapons or other stuff.
Thats the OPFOR any Japanese admiral would have to worry about if he/she was tasked to blockade NCR ports.
Not counting any of the more clever US military innovations like CLEAVER cruise missile pods deployed from cargo aircraft. Or Loyal Wingman-style drones with strike packages of their own.
That adds up to a minimum of a 1700km radius of death from an NCR port like San Diego using just the F-35s with no tankers.
That goes up to >2700km should they use P-8s or similar aircraft in the antishipping role.
The minimum survivable surface action group in such a situation just gets bigger and bigger. And requires multiple carriers.
Any single, or small group of blockading IJN surface ships in the area will simply die the moment the Japanese govt declares a blockade.
And its going to be faster to build replacement aircraft than to build replacement long range frigates/destroyers/cruisers.
Note that Im assuming Cali has no submarines either.
Or armed underwater USVs.
Because that vastly complicates the tactical picture further.
EDIT
Also, it goes without saying that in such a situation they just lost access to every military installation on the west coast of North America.
And every economic investment in the same area.
