Blueprint sections are pre-created sections that are used (and filled with content) later. You can create a blueprint in the WordPress admin and going to Section Templates -> Add new. Here you can create a new post with your blueprint section. This section can be used in two ways.
The blueprint doesn’t have it’s own default classes, because it gets used to quickly build new structured content sections. Therefor it doesn’t need to have it’s own UI or template classes. What it does need is it’s personal admin handler and a simple base-class: these makes sure that, whenever you load a blueprint; it get’s turned into a regular content section.
Admin handler class: .
If the original section is -for instance- a container; the newly created blueprint will also be of the container type. It will also add all the containered sections you might have added.