Preheat oven to 350°F. Place the butter in a 9x13 inch baking dish and set it in the oven until melted and sizzling, about 5 minutes.
In a bowl, toss blueberries with 0.25 cup sugar, lemon juice, and cinnamon; set aside.
In a separate bowl, whisk Bisquick, 0.75 cup sugar, milk, vanilla, and salt until smooth.
Remove the hot pan and swirl to coat with melted butter. Pour the batter evenly into the pan over the butter; do not stir.
Spoon the blueberry mixture evenly over the batter. Do not stir.
Bake until the top is golden and the edges are bubbly, 40 to 45 minutes. Cool 10 minutes before serving.
Notes
If using frozen blueberries, do not thaw; toss straight from the freezer and add 5 minutes to the bake time. Leftovers keep covered at room temperature for 1 day or refrigerated for 3 days. Rewarm in a 325°F oven until just heated through.