To permanently delete your blogs you have empty them first of all. Here's a quick guide on how to do it.
Once done this, go to My Sites>Settings, scroll till the bottom of the page and select Delete Site. On the next screen, there will be a yellow bar confirming the address of the blog you're about to delete. Click the Delete Site button again.
Confirm the deletion by clicking the final Delete Site button.
Please note that:
- Only the Blog administrator can delete the blog(s)
- Once you confirm the deletion, you will never be able to come back and you won't be able to you your blog's URL (es. yourblog.wordpress.com) again.
Hope all is clear. Please get in touch if you need any further support.