Your blacklist becomes someone else's favorites list.

Changing a single byte changes the entire hash, so blacklists aren't a great solution if censorship is what you're after.

Blacklists are double hashed, so they aren't useful for finding 'bad' content.

And who can assure that these blacklists won't be manipulated for someone's gain? Ie, surpressing activity they don't like on the network?

Yep, and even the content you've explicitly accessed, it's garbage collected unless you pin it. So that helps further mitigate the issue, in case you mistakenly stumble upon undesirable content.