Blue Key (Wario: Master of Disguise)
From the Super Mario Wiki
The Blue Key is a key found in Wario: Master of Disguise. It is one of three colored keys found in Allergia Gardens, the other two being the Yellow Key and Red Key. The purple treasure chest containing it is in the top-right corner of a room with a pile of blue blocks accessed using the Red Key. The chest is reached by pressing a mushroom switch to temporarily wither a vine blockade, then using Arty Wario to draw blocks on top of the blue blocks. The Blue Key opens a locked door of the same color.