What Is the Purpose of a “Blank Block” in the PoA Mechanism?

The "blank block" is the output of the PoW phase and contains only the header information, not the transaction data. Its purpose is to prove the miner expended computational effort (work) to secure the block creation slot.

This PoW proof ensures that the block generation is scarce and costly, preventing spam and deterring a single entity from monopolizing the block creation process before the PoS phase begins.