Blockchain is not the best answer for storing large blobs of data.
Classified information should remain in government custody and control at all times. Putting it in the cloud is obscene, especially if that cloud is operated by a private sector contractor. Think of what Congress has to go through to view classified information, they use a SCIF.
Why are we going to hand over that sort of information to Amazon Web Services, Google Cloud, and/or Microsoft Azure?
If the brass at DLA are so enamored with "the cloud", why don't they just build their own?