I'd make it 1 turn/aura point spent, and once used (i.e., if it was
effective in deflecting the hit), it would go away. After all nobles are
less likely to keep to righteous ways than simple folk <G>.
Curse? Curse would be a chance of a hit on said noble doing greater
damage (say 50% chance that a hit would do 21-120 rather than 1-100).
Bless and Curse would nullify each other.