How will this apply to N.Korea?
Its hard to see how it would apply. The two local actors most likely to act against North Korea other than ourselves is South Korea and Japan. And as you point out we already back South Korea. We are talking about substantially arming Japan, though Japan's goals are still defensive. The analogy might be to encourage the more militant elements in Japan. Though I'm not sure we have that much influence over Japan for it to matter.
But anyway your original claim is we didn't do anything in those other cases and you can see we did.
What do you imagine we will do against (nuclear armed and sitting on the border of China right next to Russia) Korea?
If our goal is not to annoy them then we either have to shift Japan to being more responsive or China to being less protective. I can think of things we could offer China for them to sell out North Korea. Heck I'm not sure if we promised an orderly dismantlement they might not go for it. China sends out very mixed signals when N Korea acts up.
As for nuclear armed in a real war I have serious questions about how much damage their nuclear program is capable of, though there is always risk. North Korea is a dangerous foe.
We'd blame them and condemn them and attempt to get sanctions. We already do all those things so it's an empty threat.
We don't have a full on blockade. Though a full on blockade would likely mean ship to ship battles and they could respond with attacks against South Korea. So escalating to blockade we have to be at least willing to have a war.