In current pre-releases, chat messages now also sign the previous messages, in order to make it impossible for reports to be faked by adding or removing messages in the context.

When i got a block that cryptographically signs a reference, hash or content of a previous block, resulting in an infinite ever growing securely signed chain.... this is called a blockchain.

Definition:

A blockchain is a growing list of records, called blocks, that are securely linked together using cryptography

According to the upcoming changes to the Usage Guidelines:

blockchain technologies are not permitted to be integrated inside our Minecraft client and server applications

I want to emphasize that having miners compete for who is allowed to add the next block and provide decentralization, like it is used in cryptocurrencies, is not a requirement for being a blockchain.