Preheat oven to 350 degrees. In a mixing bowl, whisk together flour, baking powder and salt, set aside. In a large mixing bowl, using an electric mixer, blend together melted butter, vegetable oil and granulated sugar until well blended, about 30 seconds. Stir in egg, vanilla and lemon zest. Blend in sour cream. Add in flour mixture alternating with buttermilk in two alternate batches, blending until combine after each addition. Fold in frozen blueberries. Divide mixture evenly among 12 paper lined muffin cups, filling each one about 3/4 full. Bake cupcakes 20 - 25 minutes until toothpick inserted in center comes out clean. Allow to cool completely then frost cupcakes with Lemon Buttercream Frosting and drizzle with chilled blueberry sauce. For best results, serve immediately after drizzling with blueberry sauce. Store in an airtight container.
Lemon Buttercream Frosting
Combine all ingredients in a large mixing bowl, using an electric mixer blend together until smooth and fluffy about 1-2 minutes. Tint with yellow food coloring if desired.
Note: if you don't want the piled high style of frosting (if you don't like a lot of frosting) then half a batch would be enough. Personally I don't normally like a lot of frosting but this frosting is so good that I liked the piled high style for these, plus it made for prettier piping with the blueberry sauce =).
Blueberry Sauce
In a medium saucepan, whisk together sugar and cornstarch. Mix in blueberries and water. Bring mixture to a gentle boil over medium heat, stirring frequently. Once mixture reaches a boil, reduce heat and simmer 3 minutes, stirring frequently. Pour mixture into a blender or food processor and blend until smooth (I blended about 30 seconds on low speed in the blender). Cool mixture completely in the refrigerator or freezer before drizzling over cupcakes.
Note: I made this recipe to be more than enough sauce for drizzling the cupcakes. I liked it so much that I piped some of the sauce over the top then I served the cupcake with additional blueberry sauce. I loved it just that much =), but if you don't want that much blueberry sauce then half of the batch listed would be enough. You could always make the full batch and if you have left over you could use it for pancakes, smoothies or drizzle it over your favorite Greek yogurt (or just eat it by the spoonful its basically just pureed blueberries, yum).