That might just depend on what parts of a "SQL database" you'd implement using a blockchain, and on the application type. You could e.g. segregate the data and only use the blockchain to authorize or timestamp transactions, for instance. In the latter case, Blockchain is just another consensus layer, and all distributed databases need one. See also https://github.com/pixelspark/catena, which uses the blockchain like a replication journal, logging only mutating queries and providing authorization based on public key crypto (disclosure: I am the author).