one time pad using blockchain
The requirement for a one time pad to be secure is that it consists of uniformly random data, only known to the participants that should be able to encrypt/decrypt.

The blockchain is not random (it's created by miners and users creating transactions), certainly not uniformly random (blocks and transactions have a very strict syntactic structure), and worst of all it is designed to be known to the entire world (or in the case of private blockchains, to all those who should be able to audit).

Short answer: bad idea.