Preheat the oven to 375F with the top rack in the middle of the oven. Grease or line a 12-muffin pan.
Mash the bananas on a plate or in a zip bag until they have a puree-like texture. Transfer them into a large mixing bowl.
Make the batter:
To the large bowl with the mashed bananas add the egg. With a fork, break up the egg and whisk it in to the bananas. Add the granulated sugar and melted butter and mix to combine.
In a medium bowl, combine the flour, baking soda, baking powder, and salt. I like to do this separately so all the muffins rise evenly.
Add the dry mixture to the large bowl and mix until well combined. Gently fold the blueberries into the batter.
Distribute the banana blueberry muffin batter into the prepared muffin pan.
Bake:
Bake in the preheated oven for 20-22 minutes, until the tops are golden and a toothpick inserted in the middle comes out clean.
Remove the muffins from the oven and let them cool down for a few minutes before removing them from the muffin pan.
Storage:
Store in an airtight container on the counter for 1 day, in the refrigerator for up to 4 days, and in the freezer for up to 3 months.