Step 1: Prepare Blueberry Puree: In a small saucepan, combine blueberries, granulated sugar (if using), and lemon juice (if using). Cook over medium heat, stirring occasionally, until blueberries burst and the mixture thickens slightly, about 5-7 minutes. Mash with a fork or potato masher if desired. Remove from heat and let cool completely. For a super smooth frosting, you can strain the puree through a fine-mesh sieve to remove skins, though it's not strictly necessary. Ensure it is fully chilled before proceeding.
Step 2: Cream Cheese and Butter: In a large mixing bowl, using an electric mixer, beat the softened cream cheese and softened butter together on medium speed until light, fluffy, and well combined, about 2-3 minutes. Scrape down the sides of the bowl as needed.
Step 3: Add Powdered Sugar: Gradually add the sifted powdered sugar, 1 cup at a time, mixing on low speed until just incorporated. Increase speed to medium-low and beat until smooth. Do not over-mix.
Step 4: Incorporate Puree and Vanilla: Add the completely cooled blueberry puree and vanilla extract to the frosting. Beat on low speed until just combined, then increase to medium-low for about 30 seconds to ensure everything is mixed and the frosting is fluffy. Be careful not to over-mix.
Step 5: Adjust Consistency: If the frosting is too thick, add milk or heavy cream, 1 tablespoon at a time, beating until desired consistency is reached. If it seems too thin, chill the frosting for 15-20 minutes, then re-whip briefly. Use immediately or store as directed.