Preheat your oven to 350°F (175°C) and grease and flour a 9x5 inch loaf pan.

In a medium bowl, combine the softened cream cheese and 1/3 cup of granulated sugar. Beat until smooth and creamy.

Add the egg yolk and vanilla extract to the cream cheese mixture and beat until well combined.

In a small bowl, combine 1/2 cup of all-purpose flour, 1/4 cup of packed light brown sugar, and 1/4 teaspoon of ground cinnamon for the crumble topping.

Cut in the cold, cubed butter until the mixture resembles coarse crumbs and refrigerate until ready to use.

In another medium bowl, whisk together the 3 cups of all-purpose flour, baking soda, and salt.

In a large bowl, cream together the softened butter and 1 1/2 cups of granulated sugar until light and fluffy.

Beat in the eggs one at a time, ensuring each is well combined before adding the next.

Stir in the vanilla extract, and then gradually add the dry ingredients, alternating with sour cream.

Gently fold in the fresh blueberries.

Pour half of the batter into the prepared loaf pan and smooth evenly.

Spoon half of the cream cheese mixture over the batter and swirl with a knife.

Pour the remaining batter over and repeat the cream cheese swirl on top.

If using, sprinkle the crumble topping evenly over the batter.

Bake for 55-65 minutes, or until a skewer inserted into the center comes out clean.

Allow the bread to cool in the pan for 10-15 minutes, then transfer to a wire rack to cool completely.