Prepare the pan: Grease a 9×5-inch loaf pan with butter or non-stick spray. Line with parchment paper, letting some overhang on the sides for easy lifting later.
Mix the dry ingredients: In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.
Cream butter and sugar: In a large mixing bowl, beat softened butter and sugar with a hand or stand mixer on medium speed until light and fluffy, about 2–3 minutes.
Add the eggs: Beat in the eggs one at a time, mixing well after each addition. Stir in vanilla extract.
Add sour cream: Mix in the sour cream (or Greek yogurt). The batter will look slightly thick and creamy.
Combine wet and dry: Gradually add the dry ingredients to the wet mixture, mixing on low until just combined. Do not overmix, stop as soon as no flour streaks remain.
Prepare the fruit: Toss blueberries with 1 tbsp flour to prevent sinking. Gently fold the blueberries and chopped peaches into the batter using a spatula.
Fill the pan: Spoon the batter into the prepared loaf pan, spreading evenly with a spatula. Tap the pan gently on the counter to release air bubbles.
Bake: Place in a preheated oven at 350°F (175°C). Bake for 50–60 minutes, or until a toothpick inserted in the center comes out clean (a few moist crumbs are fine).
Cool: Let the loaf cool in the pan for 10 minutes. Lift it out using the parchment overhang and transfer to a cooling rack. Let cool completely before slicing for clean cuts.
Serve: Slice thick and serve plain, with butter, or even lightly toasted. Store leftovers wrapped tightly at room temperature for up to 2 days, or refrigerate up to 5 days.
