1Heat oven to 350°F. Spray 2 (8x4-inch) loaf pans with baking spray with flour.
2In large bowl, beat Pound Cake ingredients with whisk until well combined. Divide batter evenly between pans. Bake 20 to 25 minutes or until cake is set in center. Cool in pans slightly, about 15 minutes.
3In 10-inch skillet, melt 2 tablespoons butter. Add rhubarb; cook over medium-high heat 5 to 7 minutes or until rhubarb is softened. Remove from heat. Stir in agave nectar, blackberries and vanilla. Cool 5 minutes.
4Cut pound cake into 1-inch-thick slices; place on individual serving plates. Top each slice with Blackberry-Rhubarb Sauce and whipped topping. Sprinkle with powdered sugar.