Blueberries do, in fact, belong in banana bread, especially when it is perfumed with cardamom. This recipe came originally from a neighbor, Les Robertson. Look in the notes to find changes I've made to make it gluten-free and have a lower glycemic index.
Prepare two loaf pans by lining them with parchment paper and greasing the part of the pan not covered by the paper.
Cream the butter with the sugars with a hand mixer or in the bowl of stand mixer.
Mix the eggs, buttermilk, and bananas and add to the creamed sugar mixture.
Combine flour, baking soda, salt and ground cardamom and mix with the wet ingredients.
Stir in the blueberries by hand being careful to not break them.
Divide the batter between the two loaf pans.
Sprinkle 3-4 tablespoons of brown sugar down the center of each loaf.
Bake 65-70 minutes.
Notes
To make it gluten-free, substitute a 1:1 gluten-free flour blend for the flour. To lower the glycemic index, use coconut sugar in place of the brown and granulated sugars. This will also change the flavor a bit because coconut sugar has more of a caramel flavor. Personally, I find it to be an improvement.