It's not a public blacklist, but this thingy[0] from another comment seems to tell you what's up with a specific site if you give it a URL. At least, I think it does, since you have to sign in with Facebook.
This is typically used for anti-spam/anti-malware. FB has a lot of automation to try to stop obviously malicious content. Just, sometimes, it hits things that aren't malicious. I'm most familiar with the malicious links that, when you click them, cause you to spam your friends with the same link.
People don't notice this automation exists, except when it doesn't work. Then they either complain "Why didn't XYZ get caught! This is obviously spam! I could write better automation." or "OMG they banned dreamwidth!".
It's possible they extended this (or something similar) to filter out sites with hate speech/"community guideline violations". And in that case some sub-domain of dreamwidth posted something horrible. And then the automation slammed the hammer on the whole domain.
