On the old blocks plugin we had a custom API endpoint for checking Blocks Usage. This is used by an external sevice to generate a report.

That's the code from the old plugin:
https://github.com/greenpeace/planet4-plugin-blocks/blob/develop/planet4-blocks.php#L137-L291

Here is how it looks (see at the bottom for p4 blocks)
https://www.greenpeace.org/korea/wp-json/plugin_blocks/v1/plugin_blocks_report/

Tasks

• Re-implement the same kind of API endpoint on the gutenberg-blocks api