What is the name of the user capability to read a reusable block?

As of now, there is no dedicated user capability to read a reusable block;
it is not possible to customize the capabilities (reading, editing, deleting) exclusively for reusable blocks.

The capabilities are currently tied into in the capabilities of the post type ‘post’ so the capabilities that you give to the post type ‘post’ will also be used for reusable blocks.