Preheat the oven to 180C/350F and line a 12 hole muffin tin with liners. I used jumbo liners and got 10 out of this recipe; it will make 12 regular size muffins.
Place flour, poppy seeds, baking powder, bicarb, salt, sugars, blackberries and lemon zest into a large bowl and mix gently to coat all of the blackberries in flour. Leave to one side.
Place eggs, vanilla and milk into a jug and beat together with a fork. Add in the slightly cooled melted butter and stir briefly.
Pour the milk mixture into the flour mixture and add in the lemon juice. Gently fold the mixture until there are no lumps of flour. Be very careful not to beat or mix too vigorously as the blackberries will break!
Divide the mixture between your liners; filling them 3/4 of the way for jumbo muffins, or level for regular muffins and place in the oven for 15-20 mins for large muffins and 12-17 mins for regular muffins. Muffins are done when they're risen, golden on top and an inserted skewer comes out clean.
Leave to cool in the pan for 15 minutes before transferring to a wire rack to cool completely.
Lemon and Blackberry Poppy Seed Muffins will keep in an airtight container, at room temperature, for 4 days. You can eat them at room temperature, or heat them in the microwave for 30 seconds and serve with butter!