Combine 1 ½ cups blueberries, 1 tablespoon sugar, and 1 tablespoon lemon juice in a medium saucepan over medium-low heat. Cook for 10-12 minutes until softened into a thick puree.
Strain the mixture through a fine-mesh sieve into a bowl, discarding the skins and seeds. Allow to cool slightly.
Return the strained juice to the saucepan and simmer for 5-7 minutes until reduced by half. Cool completely.
Beat 1 cup unsalted butter in a stand mixer for 2-3 minutes until light and fluffy.
Gradually add 4 cups sifted powdered sugar, mixing on low then increasing to medium until smooth.
Drizzle in the cooled blueberry reduction and mix on low. Increase speed and whip for 1-2 minutes until smooth and pastel-colored.
If not using immediately, store in an airtight container, refrigerating for up to a week or freezing for up to three months.