It doesn't work that way. The hashes can simply indicate proof of tampering, so instead you get (some) assurance that there was no tampering in the evidence.

Attackers can still modify the data. For example, if they modify the feed before the hash of the feed is introduced into the blockchain.

Edit; Security at that level is extremely difficult as their adversaries are much more powerful and well funded. I can understand in that respect why DARPA would want to invest a large amount into that area.