Preheat your oven to 350°F (175°C) and line a cupcake pan with paper liners.

In a medium bowl, whisk together flour, baking powder, baking soda, and salt.

In a large mixing bowl, beat softened butter and sugar together on medium speed until light and fluffy (about 3-5 minutes).

Mix in lemon zest, then add eggs one at a time, ensuring each is fully incorporated. Add vanilla extract and mix until combined.

Gradually mix in sour cream and freshly squeezed lemon juice until smooth.

Slowly add flour mixture to wet ingredients, mixing just until combined. Gently fold in poppy seeds.

Divide the batter evenly among the cupcake liners, filling each about 2/3 of the way full.

Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean. The tops should be golden brown.

Allow the cupcakes to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.

While the cupcakes are cooling, prepare blackberry frosting by blending blackberries in a food processor until smooth. Strain if desired.

In a large bowl, beat softened butter until creamy. Gradually add powdered sugar, salt, blackberry puree, and cream as needed, beating until fluffy and well blended.

Once cupcakes are cooled, frost them generously with blackberry frosting.