Preheat the oven to 350°F (175°C). Prepare a 9x13-inch baking dish by spraying with nonstick spray or buttering and lightly flouring it, then shake out any excess flour.
Prepare the vanilla cake batter according to the vanilla cake recipe instructions. Pour the batter into the prepared baking dish and spread it evenly.
Scatter the fresh blueberries evenly over the cake batter in the pan.
Make the crumb topping: in a medium bowl, whisk together 1 cup all-purpose flour, 1 teaspoon baking powder, 1/4 cup brown sugar, and a pinch of kosher salt.
Pour the 6 tablespoons melted butter into the flour mixture and gently press and toss with a fork or spatula until coarse crumbs form.
Sprinkle the crumb topping evenly over the blueberries.
Bake in the preheated oven until the top is golden brown and a toothpick inserted into the center comes out clean or with a few moist crumbs, about 30 minutes.
Allow the coffee cake to cool in the pan for about 10 minutes before slicing and serving.