Preheat the oven to 350ºF and set the rack in the bottom third of the oven. Grease a 9X5 loaf pan with butter or oil. Set aside.
Add the flour, sugar, baking powder, and salt to a large bowl. Whisk well to combine.
Add in the milk, avocado oil, egg, and vanilla extract and stir well until everything is smooth and well combined.
Add in the sourdough discard and stir just until combined.
Add the blueberries to a medium bowl and add in 1 tablespoon of flour. Toss well to combine.
Add the blueberries into the batter and fold them in gently to distribute.
Transfer the batter into the prepared baking dish.
Combine 1 tablespoon of sugar and ¼ teaspoon of cinnamon in a small bowl and sprinkle on top of the loaf.
Bake in the preheated oven for 55-75 minutes, or until a toothpick comes out clean when inserted into the center.
Allow to cool in the pan for 10-20 minutes before removing the loaf and allowing it to cool completely on the wire rack. If possible, allow it to cool in the pan for as long as you can. This will make it easier to remove.
Once the loaf is cool, wrap it in a triple layer of plastic wrap to store OR slice and serve.