I searched a bit through the documentation and code and these were my findings. I thought I'd share them for others that are interested and for future reference.

Currently, there are no domains blocked, they would appear on this API endpoint: https://firefox.settings.services.mozilla.com/v1/buckets/mai...

This is the json schema for this API endpoint: https://firefox.settings.services.mozilla.com/v1/buckets/mai...

More information on the remote settings in general: AMRemoteSettings Overview - quarantinedDomains: https://firefox-source-docs.mozilla.org/toolkit/mozapps/exte...

Remote Settings documentation: https://remote-settings.readthedocs.io/en/latest/index.html

Remote Settings Devtools - where you can see all the remote settings, that get set: https://github.com/mozilla-extensions/remote-settings-devtoo...