Out of genuine curiosity, where do blockchains enter into the messaging space? Auditable chat histories and identities? As opposed to other cryptography/security techniques, blockchain doesn't seem like an especially good match for the problem space.

A messenger using the blockchain as its medium requires no staff to operate it and has nobody responsible for it. There is nobody to blame for whatever it's used for and nobody to dictate what it can and can't be used for. The group chats will simply exist and require no maintenance which is very desirable from an app point of view. Also a big positive is that you don't have to worry about the company running your messenger going out of business and then you lose the ability to use your app.

The part I'm having a hard time with is that blockchain's specific strength relative to other distributed approaches is the ledger. That's not really the crux of the decentralized messaging problem.

Unless you need the immutable ledger, other approaches solve identity, privacy, and persistence aspects of messaging just as well or better without the overhead that blockchain approaches introduce.

I can see the value in the distributed part certainly but if the storage requirements are very large due to a large volume of messages (I'd imagine any solution which caught on could quickly outpace the number of transactions on something like the ethereum or btc network), wouldn't it necessarily just end up centralized?

It's an interesting idea though.

Well https://tox.chat and https://ring.cx (now Jami) can do just that as well as https://briarproject.org.

So these do exist. Such a thing was also being proposed for Ricochet https://github.com/ricochet-im/ricochet/issues/54