Begin by preheating your oven to 250 degrees F. Line two baking sheets with parchment paper or silicone baking mats and set aside.
Add the blueberry preserves to a heat-proof bowl and microwave for 30-45 seconds. Pour through a fine mesh strainer into a bowl. Discard the solids.
In the bowl of a stand mixer, beat the egg whites, cream of tartar, vanilla extract, and salt on medium speed until soft peaks form. Add the sugar, 1 tablespoon at a time, beating on high until stiff, glossy peaks form. This will take about 5 minutes.
Drizzle about one-quarter of the preserves onto the prepared meringue mixture in a zig-zag pattern. Using a large spoon, scoop 2 portions of the meringue mixture (about 1/3 cup each) onto the prepared baking sheet. (The jam with naturally swirl into the mixture as you do this.) Make an indentation in the center. Repeat this process adding more preserves to the meringue mixture as needed. You should be able to make 12 meringues in total.
Bake for about 90 minutes or until the outsides form a crust. Turn the oven off and prop open the door with a wooden spoon. Let the meringues sit for 1-2 hours. Transfer to a wire rack to cool completely.
In the meantime, prepare the whipped cream. Beat together the heavy whipping cream and confectioners' sugar with an electric mixer on high speed for 1-2 minutes until medium peaks form.
To prepare the blueberry sauce, mix the blueberries, sugar, salt, cornstarch, and fresh lemon juice together in a medium saucepan. Bring to a boil over medium-high heat. Cook 3-4 minutes, stirring often until several of the blueberries start to pop. Remove from heat and stir in the vanilla extract. Cool completely before serving.
To serve, fill each meringue with whipped cream and top with blueberry sauce. Garnish with fresh blueberries, if deserved. Serve immediately.