See https://blockstack.org/blog/blockstack-whitepaper-part-1:

"Blockstack binds (human-readable) domains to public keys to establish ownership of domains. These domains have associated data records as well. These small bindings are stored on the blockchain and are tamper resistant. The actual payload from the data records is stored outside of the blockchain because blockchains have limited storage space and are not meant to be used as general-purpose databases."