Preheat oven to 375F
Sift flour and 1/2 tsp salt together in a bowl; set aside
In another bowl, beat 1 cup of room temperature butter and sugar together until light and fluffy. Add the eggs one at a time, beating until well-incorporated
Mix in vanilla and lemon zest
Gently beat the flour mixture into the butter mixture, 1/3 at a time. Add 1/3 flour mixture and beat, then add half of the 2.5 tbsp of lemon juice and beat, then add half the milk and beat. Repeat until you've added the entire flour mixture, 2.5 tbsp lemon juice, and 1 cup milk
Fill cupcake liners with batter 3/4 full and bake for 18 minutes - check for doneness with toothpick
Make the frosting next - allow the blueberries to thaw for 15 minutes. Combine blueberries with 1.5 tsp lemon juice in food processor or blender and pulse until mashed
Cook blueberry mixture in saucepan over medium-low heat until mixture thickens. Stir frequently
Place butter (3/4 cup, room temperature) and shortening in bowl of stand mixer and beat on low to combine
Add sifted powdered sugar (important - sift then measure) and a pinch of salt, mix to combine. It will be very thick
While mixer is running, add blueberry mixture one tablespoon at a time until the right consistency is achieved - you'll want to use nearly all of the mixture for best taste
Frost cupcakes once they've cooled