I take a earth bless purely for the reinvigoration, the protection is just a nice bonus. Astral is a perfectly good bless as it is, twist of fate and +3 magic resistance are always useful. S9 magic path is probably the best path of magic in the game as well, for winning the end game.
Air is definetly the weakest, I like adding +3 precision for A9. The E9/A9 combo bless for strong magic nations would be cool.
On units with high protection +4 prot can be good, the difference between 4 damage and 0 damage can be huge.
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Great indebtedness does not make men grateful, but vengeful; and if a little charity is not forgotten, it turns into a gnawing worm.
And on units like knights of the chalice, black templar and living pillars the E4 bless can be a very nice bonus(4 extra prot on a unit with 10 prot is nice. 4 extra prot on a 20 prot unit is great. the reinvig helps these units and your sacred mages too).
Earth is a good bless for nations with tough sacreds/those who need reinvig(niefelheim being the obvious choice) and i also like astral on alot of sacreds(mictlans sacreds, flagellants, shadow vestals and sometimes lanka's sacreds)