During the itwiki blackout for EU copyright directive on July, I learned that the extension is not used in any WMF wiki, and probably it has never been deployed. I also don't know who its maintainers are supposed to be, so I'm CC'ing @Varnent as he's the only one with +2 access on the repo. There's currently a pending patch (https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/Blackout/+/444363/) to make the extension customizable (see T198890) and, although it's not merged (again, I don't know who to add for review) that is the version for which I'm asking a review. Any change can be made there prior to merging, instead of creating new patches.
The following should be needed:
 Security review
 Performance review
 Not sure if a design review is needed.
[x] Wiki page already exists at https://www.mediawiki.org/wiki/Extension:Blackout, will need to be updated
 I guess we need some sort of #seo related assessment to avoid problems like T199252
 Creation of phab project tracked in T203227
 Side toDo: add C/I tests (extension quibble, phan, taint-check)