Evenly sprinkle the dry yellow cake mix over the sugared blueberries, covering the surface from edge to edge. Again, do not stir. You want a loose, even blanket of dry mix on top.
Slowly drizzle the melted butter over the entire surface of the dry cake mix, trying to moisten as much of the top as you can. It’s fine if a few dry spots remain; they’ll still bake up into a nice, slightly crisp topping.
Place the casserole dish on the middle rack of the preheated oven. Bake for 40–50 minutes, or until the top is golden brown and the blueberry juices are bubbling up around the edges and through a few spots in the topping.
Carefully remove the dish from the oven and let the dessert rest for at least 10–15 minutes. This cooling time helps the hot blueberry juices thicken a bit so it’s easier to spoon out.
