One implementation of it, google's, uses leveldb:
https://github.com/google/certificate-transparency
Another one out there uses postgres. It turns out that you can use traditional databases in many cases where you think you need a blockchain, and you'll be able to waste vastly less energy on proof-of-work and vastly less time dealing with the terrible mess that is "blockchain".
The reason not to use "blockchain" is that it has 200 definitions, all of them full of people trying to get rich, not getting things done.
Databases have long-since solved the problem of storing and distributing data.
Distributed stores like etcd, zookeeper, and so on have long since solved the problem of duplicating data.
Very few people need byzentine fault tolerance (due to having a large number of untrusted actors with write access), which is the only time the additional complexity blockchain includes is actually useful