This (well, both of these).
If it would have been a Level 3 or 10 or 30 chest, then it becomes a Level 4 or 11 or 31 chest, etc. Which usually just kicks the level of droploot and generated coin/gems up by +1. (It
also mean that rare drops like Ability Tomes will trigger earlier, altho' that is a rare benefit indeed.)
The biggest problem w/ Chest Blessing is that a typical Party will "never" wait. Unless you are 1) soloing, or 2) in a well-trained/disciplined (static) party, or 3) first to the chest by enough time to do your dance, someone will
~always~ rush up and open the chest before you bless it.