It isn't: it's a chain of hashes with signature. Those predate blockchains that do things like wasteful mining. An example was Surety's timestamping service. A hashchain using standard primitives is way less wasteful, supports higher transaction volume, is cheaper, and can take advantage of hardware acceleration in client and server devices.

One of reasons I oppose blockchain tech is that you get better cost-benefit ratio out of high-performance, centralized protocols with decentralized checking. We also have some of that mathematically verified for correctness down to assembly. So, they will be more secure than complex, decentralized protocols.