Custom Blocks is a very powerful concept which lets you customize and reuse Blocks. Additionally, creating Custom Blocks allows you to Swap one Block for another.
Making a Block a Custom Block allows you to reuse the same block throughout your app without having to configure a Block from scratch each time you want to use it. You can either make a new independent copy of an existing Block or mirror the Block so that all changes to one Block will also be changed in it's copied Block/Blocks.
How to turn a Block or Layout into a Custom Block
You turn any Block into a Custom Block by giving it a name
This is the Right Hand side Block configuration panel.
Any Block can be turned into a Custom Block.
You can also use Block Contextual Menu from the View Layout.
Accessing Custom Blocks
Custom Block are available in the Custom Block tab on the bottom left of the builder.
Drag & drop Custom Block as any other Block
This MyBlock will be its own block, modifying the settings will only apply to that MyBlock.
Reuse the same block
Hold down the SHIFT key as you drag the block into the View.
Changes made to the original Custom Block will apply everywhere you use the reused MyBlock in your app.
Reused blocks preserve the name of the original block because it is essentially the same block (it's an instance or symbol of the original block).
Reusing a component is extremely useful when blocks appear in multiple places in an app (exp.: Ad Blocks). By reusing a block you can edit it one place and that change will appear everywhere the block is used.
Delete a Custom Block
A Custom Block can't be deleted while it is being used anywhere in your app.
To delete a Custom Block, first remove it from all but one place in your app. Removing a Custom Block is just like deleting a regular Block, except instead of the delete option, you will see a remove option.
When the Block isn't in use in the App, you can select the Trashcan near that block in the Custom Block list