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

Beat the cream cheese, 1/4 cup sugar, egg yolk, and lemon zest in a small bowl until smooth.

Whisk together the flour, baking soda, and salt in a medium bowl.

Cream the softened butter and 1 cup sugar in a large bowl until light and fluffy. Beat in the eggs one at a time, and then 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.

Fold in the blueberries gently using a spatula.

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

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

Bake for 50 to 60 minutes, or until a wooden skewer inserted into the center comes out clean.

Cool the bread in the pan for 15 minutes before transferring to a wire rack to cool completely.