Process the sugar, pectin, and salt in a food processor until combined. Add blueberries and pulse until coarsely chopped, 6-10 pulses. Transfer to a saucepan, bring to a simmer over medium heat. Cook until thickened, about 6-10 minutes, stirring often. Stir in lemon juice. Transfer 1 ⅓ cups jam to a bowl, cover, and refrigerate for about 3 hours or overnight. Strain the remaining jam and let sit at room temperature.

Preheat the oven to 350ºF.

Grease three 8-inch cake pans and dust with flour. Set aside.

In an electric mixer, beat butter and sugar together until light and fluffy, about 3 minutes. Add eggs one at a time, beating well after each addition, then beat in the vanilla extract.

In a separate bowl, whisk together flour, baking powder, baking soda, and salt.

Alternating with buttermilk, stir flour mixture into butter mixture, ending with flour. Beat until just combined. Pour into prepared pans.

Bake until golden and a toothpick inserted into the center comes out clean, about 20-25 minutes. Cool for 10 minutes before turning out onto wire racks. Let cool completely before assembling.

In a stand mixer, beat butter and sugar on medium-high speed until light and fluffy, about 3 minutes. Add cream cheese, one piece at a time, beating well between additions. Beat in vanilla extract and 2 Tablespoons of the strained jam. Mix until incorporated. Transfer ⅓ cup of frosting to 2 bowls. Mix in 1 teaspoon of strained jam to one bowl and 1 tablespoon to the other until incorporated for three shades of frosting.

Place one cake round on serving plate and spread unstrained jam over the top. Place second cake round on top and repeat. Top with third cake round, spreading the lightest frosting over the top and sides. Starting from the bottom, spread the darkest frosting over the bottom third, medium-colored frosting over the middle third, and lightest over the top third. Blend with a rubber spatula to finish.