I think that if the unit doesn't have multiple attacks usually other blesses will be better. For larger units nature's regeneration usually is a great option. For all units with some armor earth 9 is nice, the reinvig as a bonus for you troops and maybe sacred mages. Personally the nations I've played bless with all got earth 9
water probably is better than fire for units with single attacks, certainly if you are double blessing (with earth then naturaly)
Then again most impressive blessing play I've encountered was the mictlan triple 9 with astral, fire and and... hmm not sure but I'd guess water since that makes most sense.
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