To delete your Tumblr account permanently, follow these steps:

1. Log in to your Tumblr account.
2. Click on the user icon in the top-right corner of the dashboard.
3. Select "Settings" from the drop-down menu.
4. Click on the blog you want to delete under the "Blogs" section.
5. Scroll to the bottom of the page and click on the "Delete Account" button.
6. Follow the prompts to confirm the deletion.

Note that deleting your Tumblr account is permanent and cannot be undone, so make sure you want to proceed before doing so.