Curd: Using a rubber spatula, press blackberries through a fine-mesh sieve into a heatproof bowl to extract juice (you should have about 1/4 cup juice). Discard solids.
In a medium saucepan off heat, whisk together granulated sugar and egg yolks; whisk in lemon juice and salt.
Add butter and place pan over medium-high heat. Cook, whisking constantly, until butter is melted, mixture is thick enough to coat the back of a spoon, and small bubbles form around edge of pan, 3 to 5 minutes (do not let boil). Remove pan from heat and strain through sieve into bowl of blackberry juice. Stir to combine; adjust color as desired. Cover with plastic, pressing wrap directly against surface of curd, and refrigerate until firm, at least 2 hours and up to overnight.
Meringues: Preheat oven to 300 degrees. Using a 3-inch bowl as a guide, trace 14 circles onto parchment in pencil, dividing them between 3 sheets. Transfer to baking sheets, circles-side down
In the bowl of an electric mixer fitted with the whisk attachment, beat egg whites and salt together on medium-high speed until glossy peaks form, about 4 minutes. With mixer running, add superfine sugar in three additions, increasing speed to high and beating until meringue is stiff and glossy, about 7 minutes more. Sprinkle in cornstarch, vinegar, and vanilla; gently fold to combine.
Mound a heaping 1/2 cup of meringue into centers of each circle on parchment. Using a spatula or the back of a spoon, evenly spread meringue out toward edges, creating a small indentation in center to hold curd later. Transfer baking sheets to oven; immediately reduce heat to 200 degrees. Bake until firm but not at all brown, 1 hour, 15 minutes.
Turn off oven; let meringues cool completely in oven.
Marbled whipped cream: Meanwhile, press blackberries through sieve into a small bowl to extract juice (about 1/4 cup juice). Discard solids.
In a large bowl, whisk cream with confectioners' sugar to soft peaks. Gently fold about a tablespoon of blackberry juice at a time into whipped cream to marbleize, adding more juice as needed when serving.
When meringues are cool and completely dry, top each with 1 tablespoon blackberry curd, marbled whipped cream, and berries. Serve immediately.