Preheat your oven to 350°F (175°C) and line a standard muffin tin with cupcake liners.

In a mixing bowl, cream together the softened butter and sugar until light and fluffy.

Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

In a separate bowl, whisk together the flour, baking powder, and salt.

Gradually add the dry ingredients to the butter mixture, alternating with the milk. Mix until just combined.

In another small bowl, mash the blackberries with the lemon juice until slightly chunky.

Gently fold half of the blackberry mixture into the cupcake batter, creating a swirl effect.

Spoon the batter into the prepared cupcake liners, filling each about two-thirds full.

Drop a teaspoon of the remaining blackberry mixture on top of each cupcake and use a toothpick to swirl it into the batter.

Bake for 18-20 minutes or until a toothpick inserted into the center comes out clean.

Allow the cupcakes to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.

Garnish with additional blackberries if desired.