Toss together first 3 ingredients in a large bowl.
Melt butter in a large skillet over medium-high heat. Add nectarine mixture to skillet; bring to a boil, and cook, stirring often, 5 minutes. Spoon hot nectarine mixture into a lightly greased 13- x 9-inch baking dish. Spoon Blueberry Muffin Batter evenly over hot nectarine mixture.
Bake at 400° for 25 minutes or until crust is golden brown. Serve with vanilla ice cream.
*3 (21-ounce) cans peach pie filling can be substituted for nectarines. Omit sugar and flour. Melt butter in a large skillet over medium-high heat. Add peach pie filling, and bring to a boil; remove from heat. Proceed with recipe as directed.