Those messages are sometimes misleading.. .like now, perhaps
In order to bless something, you need...
1. An artifact spell (you've got that)
2. The Cabalist Trait
3. At least one level in the Blessing skill
4. An item that can be enchanted (some can't!)
5. That item needs at least one less blessing or enchantment than your Blessing skill level
If the item already has benefits (such as a Dragon Arrow), you need a higher Blessing level to add your first blessing to it. What are you trying to bless, what does it currently have, and what's your Blessing level?
IIRC, the Armor blessing on armor, gloves, pants, and helms adds 10% to the armor value. Putting it on a ring or bracer adds 1.0 even... a far better deal
I never put anything except Armor on rings and bracers, and will save silver rings and bronze cuffs whenever I find them. Two rings and a cuff... +36 AC (+42 if you're Enlightened)? No question there...
