I think the trick is that you need to put the Historian in a pool of blessed blood, however you want to accomplish that. Blood rain is not required but it is the easiest (for example, I didn't have blood rain, so I used teleport on a character without physical armor so they hit the ground creating a poll of blood and then blessed that).
It sounds like you cast blood rain, then cast bless on the historian. This might not have blessed the blood. Try casting blood rain, then bless the blood on the ground to create blessed blood. If that doesn't trigger the cure on the historian, then teleport the historian into the middle of the pool of blessed blood.
