Heat oven to 400°F. Spray an 8x8” or 9x9” pan with nonstick cooking spray.
Stir together the cornbread mix and brown sugar in a large bowl. Stir in the milk, oil, and egg until just moistened, then fold in the blueberries carefully.
Spread batter in prepared pan. Bake 19-24 minutes, or until the edges are golden and a toothpick comes out clean. Cool at least 15 minutes before slicing and serving.
Store in an airtight container for up to 3 days or freeze for up to one month.