In a large bowl, mash the bananas with a fork until completely smooth.

Add the baking soda to the bananas, stir to combine, and let sit for 1 to 2 minutes.

Stir the melted butter, sugar, beaten eggs, vanilla extract, and salt into the banana mixture until well combined.

In a separate medium bowl, whisk together the gluten-free flour and grated lemon zest.

Gently fold the dry ingredients into the wet mixture until just combined, being careful not to overmix.

Fold in the fresh blueberries.

Divide the batter evenly into a 12-cup muffin tin lined with paper liners.

Bake at 375°F (190°C) for 20-25 minutes, or until a toothpick inserted into the center comes out clean.