What happens when contents are not in the block though? What stops witness nodes from choosing to decline accepting operations as Steem witnesses have done for other account operations in the situation with @darthknight?

As for blacklists I have little issue with them. It's up to each site to determine what they'd like to show on their own domain. As long as the data can make it to the block then I have little issue. If I find that peakd.com is censoring content that I find valuable then I'll either move to Hive.blog or spin up my own condenser. Do you think you should have to display everything a user uploads to d.tube?