Nothing. It is an impossible construct. The blockchain is a (very limited) distributed immutable storage system. It cannot run anything, and participating in a blockchain requires a minimal operating system to be present (storage/network/process management).

Operating System: - the software that supports a computer's basic functions, such as scheduling tasks, executing applications, and controlling peripherals.

Blockchain: - A decentralized and distributed digital ledger that is used to record transactions across many computers so that the record cannot be altered retroactively without the alteration of all subsequent blocks and the collusion of the network.