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.
To further Dan's comment, run this command and see all the domains requested and not blocked for the number of days in your long term database:
sqlite3 /etc/pihole/pihole-FTL.db "select domain from queries where status in (2,3) group by domain order by domain"
In my case (for a Pi-hole serving everything in the house except my devices; about 24 clients, going back 180 days), that produces 12,108 lines of output. That's a lot of domains to whitelist.
Believe it or not, there are very good reasons to do this.
I totally understand what you say, but that would not be a problem.
I think that it actually is a very good function to have.
Pi-Hole, by definition is a blocker. It's only logical to expect it to do an abosolute block for whatever reason.
Now I should mention that such a block is not done on a permanent basis (for the valid reasons you mention.) I would use it for troubleshooting.
The feature already exists. Enter this regex as a blacklist entry. Toggle it on/off as desired. When it's on, the entire internet is blocked, except for those items you have whitelisted.