Instructions
Challenge Mode
Load the game as normal, and select "Challenge Mode."
Select Princess Peach as your character, and navigate to Mario Stadium using your controller.
Head to the bubbling water on the dock at the side of Mario Stadium.
Aim your "Heart" power attack at the bubbling water and fire. A message will be displayed to confirm you have unlocked Blooper.
Tutorial Mode
Select "Tutorial Mode" at the main menu.
Complete each of the game tutorials on the "Practice" difficulty setting to unlock Blooper.
Save your game to ensure that your progress is stored for future games.