Heat oven to 375°F. Line a 12-cup muffin pan and lightly mist liners.
Beat cream cheese, powdered sugar, and lemon juice until smooth and fluffy.
Toss blueberries with a teaspoon of the flour to help prevent sinking.
Whisk remaining flour, granulated sugar, baking powder, baking soda, salt, and lemon zest.
Whisk buttermilk, melted butter, eggs, and vanilla in a separate bowl until combined.
Fold wet mixture into dry just until streaks disappear. Gently fold in blueberries.
Spoon batter to fill cups halfway. Add a heaping teaspoon of cream cheese filling.
Cover with remaining batter. Sprinkle tops with coarse sugar, if using.
Bake 18–22 minutes, until golden and a tester shows moist crumbs, not raw filling.
Cool 10 minutes in pan, then transfer to a rack to finish cooling.