To add to this, note that the central authority does not need to be trusted beyond Bitcoin's "incentive-compatible" assumption. Consider e.g. a public Merkle tree (I believe Cryptolog runs one of those): a dishonest authority could refuse to accept your messages (and your money), but changing the past would be easily detected. (Consider storing a signature from the authority on "as of $TODAY, the root of the Merkle tree is $HASH".)
You need something like the blockchain if you demand a fully distributed solution, but it's easy enough to build a system which does not need a trusted central party.
You need something like the blockchain if you demand a fully distributed solution, but it's easy enough to build a system which does not need a trusted central party.