Prep the Oven: Preheat your oven to 340°F (170°C). Use the "conventional" setting (top and bottom heat) for even cooking.
Mash & Mix: In a large mixing bowl, peel the bananas and mash them thoroughly until smooth.
Combine: Add the eggs and peanut butter to the mashed bananas. Whisk or stir until the mixture is well combined and relatively smooth.
Fold in Berries: Gently fold in the blueberries. Try not to over-mix so you don't crush the berries and turn the batter purple.
Fill the Tins: Lightly grease a muffin tin or use paper liners. Pour the batter into the cups, filling them about 3/4 of the way full.
Bake: Place in the center rack and bake for 20 minutes. The muffins should be set to the touch and slightly golden on top.
Cool & Serve: Allow them to cool in the pan for 5 minutes before transferring to a wire rack. Enjoy them warm!