Preheat your oven to 325°F (163°C). Line a 12-count muffin tin with paper liners or lightly grease with nonstick spray. Set aside.
In a large mixing bowl, beat the cream cheese and butter together on medium speed using a hand mixer or stand mixer until smooth and creamy, about 2 minutes.
Add the granulated sugar and beat until the mixture is light and fluffy, about 2–3 more minutes.
Add the eggs one at a time, mixing well after each addition and scraping down the bowl as needed.
Mix in the vanilla extract until fully combined.
In a separate medium bowl, whisk together cake flour, baking powder, and kosher salt.
With the mixer on low speed, add the dry ingredients to the wet mixture in two additions, alternating with the buttermilk. Mix until just combined.
Gently fold in the blueberries with a spatula, being careful not to crush them. If using frozen, no need to thaw.
Divide the batter evenly among the muffin cups, filling each about ¾ full. Sprinkle tops with coarse sugar if desired.
Bake for 28 to 32 minutes, or until the tops are lightly golden and a toothpick inserted into the center comes out clean.
Cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.