Prep the pan. Heat the oven to 350°F, 175°C. Grease a 9 x 5 inch loaf pan and line it with a strip of parchment that hangs over the long sides for easy lifting later. A quick spritz on the parchment helps too.
Make the cream cheese ribbon. In a medium bowl, beat the cream cheese until smooth. Add sugar, egg yolk, vanilla, lemon juice, and cornstarch. Mix until creamy and lump free. Set aside. If it looks a touch loose, give it a minute to sit while you make the batter, it will thicken slightly.
Mix the dry ingredients. In a large bowl, whisk together flour, sugar, baking powder, and salt. A minute of whisking saves you from sifting, and it aerates the mixture a bit.
Mix the wet ingredients. In a separate bowl or large measuring cup, whisk the eggs, oil, milk, sour cream, vanilla, and lemon zest until smooth. It should look uniform, no streaks.
Combine. Pour the wet mixture into the dry. Stir gently with a spatula until just combined. A few small flour streaks are okay. Overmixing leads to a tougher loaf, so keep it chill.
Fold in berries. Toss the blueberries with 1 tablespoon flour, especially if using fresh berries. This helps them hold in place. Fold 1 cup of the berries into the batter. Save the remaining 1/2 cup for the top.
Layer the loaf. Spoon about half the batter into the prepared pan and smooth it. Dollop the cream cheese mixture over the batter in a thick ribbon, leaving a small border at the edges. Spoon the remaining batter on top in gentle mounds, then spread to cover. Sprinkle the remaining berries over the top and lightly press them in so they are just nestled, not buried.
Bake. Set the pan on the middle rack and bake for 55 to 65 minutes, until the top is golden and a toothpick inserted into the bread part (avoid the cream cheese) comes out with a few moist crumbs. If the loaf is browning quickly, tent loosely with foil at the 45 minute mark. A kitchen will smell like blueberry pie by then, which is always a good sign.
Cool. Let the bread rest in the pan for 15 minutes. Use the parchment to lift it out and set it on a wire rack to cool completely, about 1 hour. The cream cheese needs that time to set fully, so try not to rush it. I know, the wait is the hardest part.
Glaze, if you like. Stir powdered sugar with lemon juice or milk until smooth and pourable. Drizzle over the cooled loaf. Slice with a sharp serrated knife for neat pieces.