Once a block shows up for editing in the builder, switch to the general tab. You'll see the check boxes that are circled in this screenshot.
Each of these checkboxes controls an operating system's inclusion of in the app. For example, as these are set above, Android and HTML5 apps will include this block but iOS builds will not.
Any time a new block is added to an app, all these boxes will be checked.
One final note: if you change the setting on a Custom Block, as always, it will modify the setting for all instances of that block throughout the app.