This is one of those secrets I wish I would have discovered years ago because it really opens a world of possibilities within the CMS. This method was revealed to me by Brian Alson. There’s a few other ways of creating custom blocks types like this, but this one is by far the easiest.
If you’re creating a block that will contain “child” records, such as a slideshow, then I would suggest extending the child table from “content_link” table and either create a M2M table or a parent reference field back to the content block.
I would love to hear your comments or feedback if you guys find this useful.