Why Facebook block domain
The most reason why Facebook why a website or domain might be blocked by Facebook is that posting of link too many times in a promotional manner by the same person.
This is how to unblock domain from Facebook
It will show a message we couldn’t post this
Hi There ! I have tried to share a link room this website ( Your domain.com) but unable to share the post on Facebook please review this post and allow me to publish my post on Facebook.