Yes, a Japanese destroyer in SZ 16.
You’ve got three responses available to the US that I am aware of:
1: Move your entire fleet into SZ 16 to kill the blocker. Units can also come directly from the US west coast to SZ 15, meaning everything can converge on SZ 6 in the following round (or to Wake/Hawaii if you somehow scare them away (or perhaps midway if they put a naval base there)). If nothing else, you know for sure they’re coming.
2: Sending planes to kill it. 1/3 chance they lose a 10 IPC plane. Basically a 3/3 chance Japan loses an 8 IPC destroyer.
3: Actually sending a submarine or destroyer to kill it along with planes. (This is not a very smart play if America has enough planes for #2 alone to be viable, but some people don’t realize this.)
(4: Staging on Midway on a new naval base instead, but like I said, this is kinda silly for America and great for Japan unless they weren’t able to hit your destroyer block on the turn before for some reason.)
#'s 2 and 3 are very annoying, and not great for Japan, and are obviously not very sustainable, so this is why the destroyer block is something you should only do when necessary.
This is why I like to keep 1-3 fighters (usually 3) on Japan proper and build, or at least keep, one or two boats there per turn. If America does want to come swooping in to SZ 6, they are going to need enough to get through 1-2 boats and fighters. This means they must either over-invest (because you didn’t scramble), or they under-invest and then you kill them for (hopefully) a decent ratio when you do scramble. If they over-invest, kamikazes are an option (see edit), but maybe more importantly it means that whatever they sent to stand up to your boats/planes is now within range for a super nice counterattack with subs/planes.
Obvious times when the destroyer block is worth it:
-America has enough transports on Hawaii to make Japan worry about its capital, or potentially Korea.
-Japan built a lot of navy (more than one or two boats) on SZ 6 and wants to keep it alive.
Moreover, buying 1-2 boats a round off of Japan and keeping planes there means that if america tries to pre-block you in SZ 16 (moving in, say, some amount of destroyers to SZ 16 and SZ 25 while his fleet is coming from Panama/Sydney, to in effect block your attempts to block him with only a simple destroyer of your own), you have boats and planes available to you to destroy his block-blocker in SZ 16 and remain able to delay him for a round.
I keep 1-3 fighters there starting from J1, and begin building 1-2 boats there J3+ in most instances.
Last bit of advice: If you think there’s a good chance America is going to bring in a lot of navy on its noncom to SZ 6 after attacking it with a smaller force (or they telegraph this intention by bringing a lot of planes to the fight in SZ6), and Japan is sitting on FIC/Philipines with all of its navy/air in range to attack on the very next round, do not be stingy with your Kamikazes. If you can destroy even one more destroyer before the big navy clash, that can swing the probability by something like 5 (+/- 2)%. The entire Pacific depends on a massive fight such as this. 1% would be worth it, let alone more, and you might even hit multiple boats if you’re a lucky b��t��d. When it’s crunch time, smoke 'em if you got 'em.