When text is added to a page, a block is required. The block will be in the block list and will be shared. However, most of the text blocks will not be shared and it takes a lot of clicks to add the block.
When editing a page, next to the "Add block", add an "Add text block" button
Click on the button will open a modal dialog for a new text block
The modal dialog will function like
When saving the block, the block will not be visible in the block overview or insert block dialog (the block cannot be shared)
When removing the block from the page, it need to be removed from the database as well (on save)
Maybe set a field "parent" to page-pageid to indicate a block to be for one page only. In the block overview only blocks without a parent will be shown.
Open modal dialog:
Create text block: