Preheat the oven to 350°F (175°C). Grease a 9×5-inch loaf pan with butter or non-stick spray, then line it with parchment paper for easy removal.

In a large bowl, whisk together the flour, baking powder, and salt; set aside.

In another bowl, beat the butter and granulated sugar until light and fluffy. Add the eggs one at a time, beating after each addition. Stir in the vanilla extract.

Gradually add the dry ingredients, alternating with the milk, mixing gently until just combined. Be careful not to overmix, as this can make the bread dense.

Toss the blueberries with 1 tablespoon of flour to prevent them from sinking. Fold them into the batter gently.

In a separate bowl, beat together the cream cheese, powdered sugar, and lemon zest (if using) until smooth.

Pour half of the batter into the prepared loaf pan. Spread half of the cream cheese mixture over the batter. Add the remaining batter, then swirl in the rest of the cream cheese with a knife to create a marbled effect.

Bake for 50-60 minutes, or until a toothpick inserted in the center comes out clean. If the top browns too quickly, loosely cover it with foil during the last 15 minutes of baking.

Let the bread cool in the pan for 10 minutes, then transfer it to a wire rack to cool completely before slicing.