Spray a large bowl with cooking spray. Add the cereal and set aside.
Spray the baking pan and set aside.
In a medium-large bowl, place the marshmallows and butter. Heat in the microwave on high for 1 minute. Stir the marshmallows and heat for another 15 seconds. Let the mixture sit in the microwave 1 minute.
Stir very well until the mixture is smooth, using the residual heat to fully melt the marshmallows. Stir in the blueberries.
Pour the marshmallow mixture over the cereal. Gently stir until thoroughly combined.
Transfer to the baking pan, pressing evenly.
Let set for 1 to 2 hours, or chill more quickly in the refrigerator.
Cut into 16 or 25 squares.
Notes
TIP: To prepare on the stove top, place the butter and marshmallows in a saucepan set over low heat. Stir frequently. When about ¾ of the marshmallows are melted, remove from heat and continue to stir until very smooth.