From @JustinDrake on Thu Dec 22 2016 12:43:53 GMT+0000 (UTC)
Answer: base32encode-nopad(base58-decode(HASH)), but this will change soon (see Default 5 digit prefix for flatfs is no longer sufficient for CidV1 路 Issue #3463 路 ipfs/kubo 路 GitHub)
Copied from original issue: What is the mapping between block hashes and storage in .ipfs/blocks/? 路 Issue #213 路 ipfs-inactive/faq 路 GitHub