Just wanted to provide some info on what I found. I tried both the poison and God's Wrath approach. First off, I would like to simply mention that the poison doesn't just do 10-30 damage, rather it does around 300 damage total, possibly more. The damage only registers when the blue dragon lifts its tail. This brings me to the second bit of information. The reason it doesn't hit 100% of the time is because damage can only be inflicted when the tail is getting ready to do a tail smash. Just time the tail smash correctly so when that tail lifts God's Wrath goes through its animation.
Blue Dragon / Discussion
This is the discussion related to the wiki page Blue Dragon.