Maybe it was a bad example to talk about messaging. It's not just the "is this data really who they claimed to be", but also "is this data part of transaction between multiple parties who do not trust each other"?

You need to have distributed consensus to avoid double-spending. Can you have distributed consensus without blockchain? Yes, but only if your participants are selected a priori, e.g, Distributed Paxos.

The problem that blockchains solve is that it is the only practical way to solve consensus between parties that do not trust each other and that are not pre-agreed on.