Which URLs are blocked and who blocks them?

A couple of times I've tried making posts and got an error of "Lemmy: blocked URL". As the first time it happened was with a post with multiple links, I didn't know which site was the problem.

The second time with just one link, "Oh, it's Blogspot." (IIRC, a completely different blog, and on a different /c.)

• How can I find out which URLs are blocked, so as to not waste my time trying to post them?
• Who is it blocking them?
• • individual communities?
• • the instance of the community?
• • my instance? (lemmy.ca)
• • all Lemmy everywhere all at once?
• I guess I'm out of the loop, why is Blogspot blocked?