Preheat oven to 400°F. Line a muffin pan with liners and spray with oil.
Combine soy milk and apple cider vinegar in a small bowl; set aside to curdle for 5 minutes.
Whisk dry ingredients (flour, sugar, baking powder, salt, cornstarch) together in a large bowl.
Add milk mixture, oil, and vanilla to dry ingredients; stir until just combined.
Gently fold in blueberries, being careful not to overmix.
Fill muffin liners ¾ full; sprinkle with coarse sugar if desired.
Bake for 22-26 minutes until golden brown and a toothpick inserted in center comes out clean.