How can I delete a complete blog?
The same rules apply as with a page.
To delete a page:
Go to the page and choose the three dots on top of the menu and then Delete. You’ll be warned of any issues, such as incoming links that will break. Choose Delete to proceed.
All detailed info about deleting blogs and pages you can find here:
https://confluence.atlassian.com/doc/delete-or-restore-a-page-139429.html