Preheat your oven to 350°F (175°C). Grease and flour a standard 9×5 inch loaf pan.

In a small bowl, beat the softened cream cheese, 1/4 cup sugar, egg yolk, and lemon zest until smooth. Set aside.

In a medium bowl, whisk together the flour, baking soda, and salt. Set aside.

In a large bowl, cream the softened butter and 1 cup sugar together until light and fluffy.

Beat in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

Mix in the sour cream or yogurt until just combined.

Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix.

Gently fold in the blueberries using a spatula.

Pour half of the batter into the prepared loaf pan. Dollop spoonfuls of the cream cheese filling evenly over the batter. Pour the remaining batter over the top.

Use a knife or skewer to gently swirl the cream cheese filling through the batter to create a marbled effect.

Bake for 50 to 60 minutes, or until a wooden skewer inserted into the center comes out clean. If the top browns too quickly, loosely tent it with foil.

Let the bread cool in the pan for 15 minutes before carefully removing it to a wire rack to cool completely before slicing.