Useful information to include:

• Minecraft version: 1.16.2
• CC: Tweaked version: 1.91.2
• Detailed reproduction steps:

1. open a new world
2. place a computer
3. run lua
4. run http.checkURL("http://localhost")
5. • Result: "Domain not permitted"
6. go to <intall_root>/config/computercraft.json5
7. remove all elements from the blacklist
8. add localhost to whitelist
9. restart Minecraft
10. re-do steps 3 and 4
11. Result unchanged (5)

Probable cause :

I've spent a few minutes looking at the code and what I believe to be the issue is that this
https://github.com/Zundrel/cc-tweaked-fabric/blob/4d00969ef0a6907e122789995ae36fa10070ccb3/src/main/java/dan200/computercraft/shared/util/Config.java#L162-L165
Enables the updating of the whitelist and blacklist, however (I think) the URLs are checked against the httpRules
https://github.com/Zundrel/cc-tweaked-fabric/blob/4d00969ef0a6907e122789995ae36fa10070ccb3/src/main/java/dan200/computercraft/ComputerCraft.java#L89-L95
which are not re-created when the blacklist and whitelist arrays are updated.

I'm happy to help if you need more information.