Ability to block all sites and then add exceptions

One of the most important features is the ability to block all websites and then create exceptions so that only a few websites are whitelisted.

copy/add the options LeechBlock offers to specify which sites should be blocked or delayed. This adds a great deal of flexibility and allows the intervention to be applied by default on all websites, not just those configured manually, while still allowing exceptions to be added.

from the LeechBlock documentation:

Enter the domain names of the sites to block (one item per line):

This field contains the list of sites to be blocked. If you don’t enter anything here, nothing will happen!
Note that you should omit any URL protocol prefixes (e.g., https://) and you don’t need to include the www prefix. (LeechBlock will block the site with or without the www prefix.)
If you want to block only part of a site, you can specify a path after the domain name (e.g., somesite.com/somepart). You can even block a specific page by specifying the exact path to the page (e.g., somesite.com/somepart/somepage.html).
You can use * as a wildcard, + to prefix exceptions, > to prefix referrers, and ~ to prefix keywords.

https://www.proginosko.com/leechblock/documentation/#BlockSetOptions