I did read
As a new Pi-Hole user, I did quite some research to come up with (find) this posted solution. I would request this feature on the Domain Manegement page (seems the logical place) where right below after the main title you can have a "Create Whole-World-Block" checkbox, and it's corresponding "create" button.
Many times I have already come with the conclusion that it's actually simpler to start in this manner, If everything and anything is blocked to begin with, to a certain Client X, then I can begin opening the specific domains (websites) this computer will see. And nothing else.
It might even be good enough to add some text that says "Use a simple * to block everything", as described in the post I mention above.