In a medium mixing bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. Set aside.

In a larger mixing bowl, beat the softened butter and granulated sugar with an electric mixer until light and fluffy.

Add the eggs one at a time, mixing well after each addition. Then stir in the vanilla extract and sourdough discard until smooth.

Gently mix in the sour cream or yogurt until just combined.

Gradually add the dry flour mixture to the wet ingredients in two or three batches, stirring gently with a spatula to avoid overmixing.

In a small bowl, toss the fresh blueberries with one tablespoon of flour until they are lightly coated.

Fold the flour-coated blueberries into the batter carefully to prevent crushing them.

In another small bowl, beat the softened cream cheese with a quarter cup of sugar until smooth. Add the egg yolk and vanilla extract, mixing until well combined.

Pour half of the blueberry batter into a prepared loaf pan, spreading it evenly.

Spoon dollops of the cream cheese mixture over the first layer of batter.

Pour the remaining blueberry batter over the cream cheese layer, spreading it to cover completely.

Use a butter knife or wooden skewer to create a zigzag swirl through the batter.

Bake in a preheated oven for 55 to 65 minutes, or until the top is golden brown and a toothpick inserted in the center comes out clean.

If the toothpick has wet batter, return the pan to the oven for an additional five minutes.

Once baked, remove the loaf from the oven and let it cool in the pan for 20 minutes before transferring it to a wire rack to cool completely.