This is the temporary name given to the microblogging application that is being built by the Leofinance team.
It is affectionately being called "Twitter on the blockchain".
The idea is to have a short form content application built on the Hive blockchain. This will provide account ownership since access is all done via private keys. Since it utilizes the base layer of that network, all data is immutable.
Proof-of-Brain is a mechanism that most applications on Hive utilize. ProjectBlank reportedly will have its own token, allowing users to be rewarded in that along with HP and HBD.
We are seeing the advancement of the infrastructure development with the upcoming release of Threads on Leofinance. This is effectively serving as the testnet for the full application.
There is no timeline on the expected release of ProjectBlank.