Preheat oven to 350°F (175°C). Line a 9x13 inch metal baking pan with parchment paper.
In a large mixing bowl, cream butter and sugar until light and fluffy.
Beat in eggs, vanilla extract, and vegetable oil until well combined.
In a separate bowl, whisk together flour, baking powder, and salt. Add dry mixture to wet mixture alternately with milk, beginning and ending with dry.
Toss blackberries with flour, then gently fold them into the batter.
Pour batter into prepared pan. Bake for 40 minutes or until golden and a toothpick comes out clean.
Cool cake in pan for 30 minutes. Whisk together glaze ingredients and drizzle over cooled cake.