CAKES: Preheat oven to 350 degrees and place liners in cupcake pan, set aside. In a medium bowl mix flour, salt, baking soda and baking powder. Set aside.
In another medium bowl, mix together the yogurt, milk, vanilla and lemon juice. Set that bowl aside as well.
In a large bowl cream the butter and sugar. Beat in the eggs one at a time, mixing well between additions. Add the lemon zest and alternate between adding the flour mixture and the lemon mixture until completely combined. Now add the poppy seeds.
Distribute evenly between cupcake liners and bake until a toothpick inserted into the cupcakes comes out clean, 30-35 minutes.
CURD: While the cupcakes are baking, you can prepare the lemon curd. Using a double boiler, whisk all the ingredient together. Allow the curd to cook for about 15 minutes stirring every few minutes until it thickens to a nice consistency. Remove from heat and allow to cool.
FROSTING: Send the blueberries through a blender or food processor then set aside. If you are using frozen, squeeze out the extra moisture before setting aside. Cream the butter and cream cheese until no lumps remain. Add powdered sugar and blueberries and mix until frosting forms.
TO ASSEMBLE: Scoop out the middle of each cupcake and place a spoonful of curd into each one. Fill a piping bag with frosting and use a wide piping tip, preferably something circular, blueberry chunks can clog your tip. You can always just spoon the frosting on top as well!