Not much! He gives the version of the Value of Information Theorem that philosophers tend to know about. Blackwell doesn't state this particularly explicitly, as far as I can see, though it's clear once you understand Blackwell that the theorem is an easy consequence of his work.
As regards priority, what does Good add to Blackwell, who (I think) was first by a couple of decades
Not much! He gives the version of the Value of Information Theorem that philosophers tend to know about. Blackwell doesn't state this particularly explicitly, as far as I can see, though it's clear once you understand Blackwell that the theorem is an easy consequence of his work.