In a large bowl, whisk sourdough starter, eggs, melted butter, sugar, vanilla, and milk together. Remove a tablespoon of flour from the 1/2 cup and add it to a medium bowl with the blueberries. Stir to coat.
Sift the flour, salt, and baking powder together.
Mix the sourdough mixture with the wet ingredients.
Fold blueberries into the muffin batter using a spatula or wooden spoon.
Divide the batter between the paper liners.
Mix the flour, baking powder, and sugars together.
Use a fork or pastry blender to cut the butter into the flour mixture so crumbs form. Pour in the heavy cream and stir to make larger clumps.
Sprinkle the crumb topping on each muffin. Add a few more blueberries if desired.
Baked for 6-7 minutes in the preheated oven, then turn the oven down to 350 degrees and bake for another 18-20 minutes.
Allow muffins to cool in the pan for 5 minutes, then transfer them to a wire rack to finish cooling.