Invisible Ink is a certified mail application that demonstrates the utility of the blockchain for maintaining a public ledger of transactions while keeping the content of those transactions private. In this case, the idea is a method for guaranteeing the delivery, receipt and existence of email messages. It archives the transaction in the Bitcoin blockchain and uses secure off-chain storage for the other details. Invisible Ink demonstrates the extensibility of this distributed technology for contracts, audits, and recovery of sensitive information. It is an evolution of work begun as the Ethos project. Since Bitcoin has shown that a distributed system of trust can be workable for irreversibly storing time-stamped information, these extensions and applications are potentially important for a wide variety of cases from finance to personal information.