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

Whisk together the all-purpose flour, poppy seeds, baking powder, baking soda, and salt in a medium bowl.

In a larger bowl, cream softened butter and granulated sugar until light and fluffy, then mix in eggs, sour cream, lemon juice, zest, and vanilla until combined.

Gradually add the dry mixture to the wet mixture, mixing on low speed until just combined.

Spoon the batter into cupcake liners, filling each about two-thirds full, and bake for 18-22 minutes.

Cool cupcakes in the tin for 5 minutes before transferring to a wire rack to cool completely.

Mash blackberries and strain for puree; beat softened butter, then gradually add powdered sugar, blackberry puree, lemon juice, and vanilla until fluffy.

Frost cooled cupcakes with the blackberry frosting and decorate as desired.