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In Paper Mario, a Blue Key appears in Crystal Palace along with a Red Key and a Palace Key. The Blue Key is found to the south of the entrance room. After getting this key, Mario will be able to open the door in the higher place. The description reads, "A big blue key."
In Paper Mario: The Thousand-Year Door, the Blue Key is located inside the Great Tree, in the room after the small Jabbi army. The key opens the cage where ninety Punis are imprisoned, increasing Mario's Puni army from eleven to one-hundred and one. Its description reads "A key that opens the blue cell door."
In Mario & Luigi: Bowser's Inside Story, a blue key can be found in the second floor of Peach's Castle. It is carried by a Fawfulcopter which Bowser chases after. In the southernmost room on this floor, Bowser must push himself as a Spike Ball against a large spring to shoot himself to the Fawfulcopter. When successful, he knocks the Fawfulcopter and gets the key which allows him to proceed further. The Star Menu description reads, "A key that opens the blue door on the second floor of Peach's Castle."
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