Lemon Blues Muffins with Crumble Topping

Name: Lemon Blues Muffins with Crumble Topping
Ingredients: 1/2 cup (1 stick) unsalted butter, at room temperature; 1 cup granulated sugar; 2 large eggs; 1 teaspoon natural lemon extract; 1 1/2 cups all-purpose flour; 1 1/2 teaspoons baking powder; 1 teaspoon salt; Grated zest of 1 lemon; Pinch of freshly grated nutmeg; 1/2 cup low-fat buttermilk; 1 cup blueberries; 1 1/4 cups all-purpose flour; 1/2 cup (1 stick) unsalted butter, melted; 1/4 cup granulated sugar; 1/4 cup packed light brown sugar; 1/4 teaspoon salt