How does ContentBlocks store the content information?

The processed content, as HTML, is stored in the resource content and can be accessed with [[*content]].

The raw content, made up from layouts, columns, and fields, is saved to the resource properties in the contentblocks namespace. 

If you're looking to programmatically import content into ContentBlocks, you'll need to prepare the structured data for the properties just right - if the data is invalid it will likely break the interface. The simplest way to do so is first to build an example page in the manager, with the right layouts and fields you want to import and some dummy content. Then you can copy the structured data from the resource properties column, and mimic that format in your import script.