Cactus isn't really the fediverse though: I consider (maybe mistakenly) the so-called fediverse to be broadly activitypub-based, while cactus leverages Matrix.

Being decentralized is orthogonal with moderation. You can for example allow people to post to some rooms that represent the blog posts threads, and keep yourself as admin and moderator there.

See https://cactus.chat, which is on the Matrix network.

Dunno about blockchain, but decentralized works great. Watch "Decentralized Social Networks vs. the Trolls" on (decentralized) PeerTube:

https://conf.tube/videos/watch/d8c8ed69-79f0-4987-bafe-84c01...

IPFS

https://komento.host/

https://news.ycombinator.com/item?id=26840855

Presumably the comments would be sharded and encrypted. Then each those chunks would have a hash generated for it, and each chunk would be stored as transactions on a blockchain, or more likely multiple blockchains for redundancy.

This is how the blockchain storage companies do it.

Decentralizing Disqus is trivial. Just go back to each blog has its own local comment system with commenter identities disconnected from all other sites they comment on or even anonymous. There's no reason you need a single global identity and storage backend for all content sources that support commenting.

It can even be otherwise identical to Disqus except self hosted.

Local storage: solves the spam issue and the issue of annoying commenters.

https://github.com/tessalt/echo-chamber-js

The same way a chat system can be. See https://cactus.chat, on the Matrix network, for example.