Preheat your oven to 350°F (175°C).
Grease a 9x5 inch loaf pan with cooking spray or butter, or line it with parchment paper.
In a large mixing bowl, combine the egg, Greek yogurt, coconut oil, light brown sugar, dark brown sugar, vanilla extract, and mashed ripe bananas. Whisk until smooth.
In another bowl, whisk together the flour, baking powder, baking soda, cinnamon, nutmeg, and salt. Gradually add this dry mixture to the wet ingredients, stirring gently until just combined.
Carefully fold in the frozen blueberries.
Pour the batter into the prepared loaf pan, smoothing the top with a spatula.
Bake for 55-65 minutes, or until a toothpick inserted comes out clean.
Let the loaf cool in the pan for about 10 minutes, then transfer to a cooling rack to cool completely.
In a medium bowl, combine the softened butter and blueberries. Mash until spreadable.
Slice the banana bread and serve warm with blueberry butter.