In Ni No Kuni 2: Revenant Kingdom, you will often come across blue treasure chests scattered around the open world. These contain better loot than the normal brown treasure chests.
By default the game does not let you open them. There’s a little something you must do first.
Here’s how to open blue chests:
You must research the spell called “Specific Spell Dev: Spring Lock” at the Evermore Spellworks (your castle). It reads: “Research into the use of “Spring Lock,” a spell that unseals the inaccessible”.
First you must build the Evermore Spellworks and upgrade it. Then research this spell. It becomes available sometime after chapter 5.
I only recall finding blue chests in outdoors regions. Do note that the blue chests in Dreamer Door Mazes are different, they require purple orbs to be opened (which drop by defeating enemies inside the Dreamer Mazes).