Morale changes aren't visible. The spell works just like a death of comrades, Fear ability or any similar morale-reducing thing would. One way to test it would be to have your side only cast Blood Rain on the enemy (no kills, no inflicted wounds, no other spells etc) and check whether the enemy priests cast morale-restoring spells (Fanaticism or the AoE version).
An old one, I got bored with the other one. No one seemed interested in the furry kingdom mod Saber Cherry once posted... And I had just the Badger Knight.
Maybe I should get back to working with certain mod, too...