One way of testing the functionality of these options is to go into the Project Settings and go to the “Engine - Streaming” page. There you can set the “Actor Initialization Update Time Limit” to a very low value like 0.1. This will cause the streaming to only add one or two objects from the streaming sublevel per second - at least on my machine.
If you only enable “Make Visible After Load” you will see each object from the sublevel appear as they are streamed in. So I guess it’s not making the objects visible after load, but rather during load. If I disable the setting it won’t show any objects from the level, even when it is done streaming.
If you enable “Should Block On Load” it seems like the node doesn’t really block anything but the execution path it’s connected to. So if you connect it to the Event BeginPlay node and you have a Print String node after it, the Print String will not execute until the level is loaded. But from what I can see, everything else (like the Event Tick or other blueprints) keeps running as usual.
So if you want everything to just stop while you’re loading I guess you could set it up so that you first use “Set Tickable When Paused” in the blueprint that you’re using for loading. Then you pause the game (“Set Game Paused”). Then you run Load Stream Level and set up an event in the Event Tick that keeps checking every frame to see if the level is loaded (Get Streaming Level → Is Level Loaded). When the level is loaded you have a branch set up that unpauses the game.
Or you could just sprinkle a few “Is Level Loaded” in select places that you don’t want to fire while loading.
By the way, make sure you’ve set up your levels correctly in the Levels menu (Windows/Levels) and in the level streaming volume settings. If you click on a sublevel in the Levels menu and click the “Summons level details” button you will see some options. One of the options is “Initially visible” which may conflict with your settings in the Load Stream Level node and cause confusion. If you’re using streaming volumes there’s an additional setting in the actual volumes that may cause conflicts as well. So yeah, there are three places where you can set visibility for streaming levels. There’s also a “SVB Blocking On Load” setting in the streaming volume, but I’m not sure how it works (i.e. what it’s actually blocking).