Hi Louis,
I'm sorry I missed this question so long ago. I hope my answer can still be useful.
Regarding documentation, it has been updated in v12:
https://reference.wolfram.com/language/guide/Blockchain.html
I also encourage you to check the featured examples here:
https://www.wolfram.com/language/12/blockchain/
Now, about the sandbox blockchain (WolframBlockchain), it is still a very experimental blockchain we support (a MultiChain instance indeed) and it served to experiment with BlockchainPut and BlockchainGet which are the blockchain equivalent functions of Put and Get.
We've been focusing our efforts on adding cool support to other blockchains like Bitcoin, Ethereum and ARK (both their mainnets and testnets/devnets).
Also, in v12.1 the documentation will be much improved.
WolframBlockchain will be kept experimental until come up with a cool design and implementation.
Please let me know if you have more questions or comments.