Preheat oven to 400 degrees. Line a 12 cup muffin tin with paper liners.
In a med bowl, combine flour, baking powder, sugar, and salt. In a large bowl, beat eggs, milk, oil, and vanilla for 1 min on medium. Add flour mixture. Beat just until no streaks of flour remain-do not overmix! Fold in cup of blackberries.
Spoon batter into muffin cups. Bake for 18-20 min or until tops of muffins spring back when pressed lightly. Let cool in tin for a minute-then transfer muffins to a cooling rack.