I have come with a slightly different conclusion about Blessing. Please bear me with my bad grammar.
I’m currently playing 2 characters, one dps hoplite and one support priest (lvling it for my gf). I surprisingly find out that blessing actually protects the usage count of concentrate - that is, the 10 hit limit of concentrate won’t be used as long as there’s blessing buff. I don’t know if this is a bug or intended but for now, i can have my concentrate 100% time on my hoplite with blessing buff and that’s considered quite a dmg booster to me.
Besides, a large portion of Blessing damage comes from its attribute (+100 with lvl50 enhance). That makes the point investment in Blessing mainly for its hit count.
Thirdly, you have to consider that Blessing does not only affect you but also your teammate, and Blessing procs multiple times on multi-hit skills - skill such as stabbing, multishot, or even cure would benefit greatly from its flat damage increasement.
Finally, in your previous calculation, defense was not taken into consideration. What if the enemy has 100 physical defense and 300 magical defense? I don’t know much about the defense of end game mobs but what’s certain is the more defense they have (or the less attack we have), the better Blessing will be, unless mobs defense are so high that we still only do ~1 damage after buff, but I don’t think that’s gonna happen?
So it comes to my conclusion that, investing in Blessing is a good idea - but not too many points. We just need enough Blessing lvl to sustain hit counts, probrably lvl5? Don’t forget we still need lvling and Blessing is quite strong in lower lvls…having Blessing lvl5 would make one’s Priest C1 life better.