Preheat oven to 350 degrees and line pans with cupcake liners.
Sift cake mix into a separate bowl and set aside.
In a large bowl combine eggs, oil, zest, lemon juice, vanilla, milk and sour cream until smooth.
Stir in cake mix.
Bake for 17-22 minutes or until an inserted knife comes out clean.
Glaze: Combine all ingredients in a bowl (using a whisk or fork is easiest).
Buttercream: Beat butter until smooth. Add enough powdered sugar to make the frosting stiff, then slowly add in blackberry puree 1 Tablespoon at a time until you reach your desired consistency.
Spoon glaze over cooled cupcakes and let set.
When glaze is firm, pipe frosting on top and finish with a fresh blackberry.