You'd think with so many previous infractions you'd be more aware of the rules.
Ok some people are mixing up between the proc of the blessing and the bubble-proc. They work in 2 different ways, so we need to look at them differently. I've read the blueposts over and over again, but I'm not 100% sure if I am right, so you might correct me if I'm wrong.
1) Proc of "Blessing of Kings":
This seems to be clear. If one of your spells actually HEAL, you got a 10% chance to proc the blessing. No matter what spell you use, no matter for how much you heal, it just needs to be a heal (say 1 hp or more). 2 Examples:
-A Paladin casts 5x Holy Light in 15 seconds. With his beacon up, he got 10 heals in sum. However, 4 of them hit the target at full health. So at the end, he did 6 actual heals, means 6 chances at 10% to proc to blessing.
-A Holy Priests casts (empowered) renew, prayer of mending, flash heal, Circle of Healing, flash heal, prayer of healing, flash heal. At best, renew ticks 6 times, PoM 6 times, 3x flash heal, Coh hits 6 people, PoH 5. That's 26 Heals in sum. However, assume 10 of those heals are vasted (100% overheal, not all people in range, etc). So he still got 16 chances at 10% to proc the Blessing.
So if there's raiddamage, Holy Priests (or druids) can pop up the blessing pretty fast compared to Paladins or Shamans, which should be somewhere in between. If there's no raiddamage, a Priest can spam CoH and whatnot, he won't be able proc the blessing.
2) Proc of the "bubble":
Once you got the "blessing of kings" on you, every cast places a bubble on the target(s) you are going to heal. The amount the bubble can absorb is 15% of the total heal done to the target no matter of overheal. A 20k Holy Light will pop up a bubble with 3k, no matter if you actually did 100% overheal or a 20k heal. Or Prayer of Healing healing 3x 4k and 2x 6k. So 3 targets receive a bubble worth 600 hp, and 2x 900, in sum 3600 hp but split up on different targets.
Every tick of a HoT will add to the bubble as well, however you need to remember, that this will just happen as long as the blessing is active. If you cast renew 2 seconds before the blessing ends, you won't see a bubble on the target as the first tick occures after 3 seconds (if you didn't skill empowered renew tho *g* ).
I think this Mace is pretty usefull for any healing class away from Disciplin Priests (sadly, can't understand why Blizzard didn't add Shields to the proc). People may discuss if it's better for MT heal oder raidheal, but as it is a Proc with an internal CD, that's no matter in my oppinion. You cannot be sure to have it in the right moment, so it makes things smoother, but if you can't win a fight without the mace, you won't win with it.