Prep the pan. Heat oven to 350°F (175°C). Grease and line a 9-inch round cake pan or springform with parchment on the bottom.
For a taller look, use a 9x5-inch loaf pan and extend bake time.
Mix the dry ingredients. In a large bowl, whisk flour, sugar, baking powder, baking soda, and salt until evenly combined.
Whisk the wet ingredients. In a separate bowl, whisk buttermilk, oil, melted butter, eggs, and vanilla until smooth.
Combine gently. Pour the wet into the dry and fold with a spatula just until no flour streaks remain. Don’t overmix.
Toss the berries. In a small bowl, toss blueberries with 1 tbsp flour. Fold 1 cup of the berries into the batter.
Reserve the rest for the top.
Make the cream cheese layer. Beat cream cheese, sugar, egg yolk, vanilla, and a pinch of salt until silky and smooth.
Layer the cake. Spread half the batter into the pan. Dollop the cream cheese mixture over it and gently spread to the edges. Top with the remaining batter, smoothing lightly.
Add berries and topping. Scatter the reserved blueberries over the top.
Sprinkle with coarse sugar and lemon zest.
Bake. Bake 45–55 minutes (round pan) or 60–70 minutes (loaf), until the top is golden and a tester inserted near the center comes out with moist crumbs, not wet batter. The cream cheese may cling slightly—that’s okay.
Cool. Let cool in the pan 15 minutes, then lift or invert onto a rack, peel parchment, and cool completely before slicing.