Combine blackberries and lemon juice in a small saucepan over medium heat; simmer for 10 minutes until softened.

Press the blackberry mixture through a fine-mesh sieve to remove seeds, return the liquid to the pan, and reduce until only 2 tablespoons of thick syrup remain. Let cool completely.

Preheat oven to 200°F (95°C) and line two baking sheets with parchment paper.

In a clean glass or metal bowl, whisk egg whites, cream of tartar, and salt on medium speed until soft peaks form.

Increase speed to high and gradually add sugar one tablespoon at a time, whisking until the meringue is glossy and holds stiff peaks.

Drizzle the cooled blackberry reduction over the meringue and gently fold once or twice to create a marbled effect.

Transfer the mixture to a piping bag fitted with a star tip and pipe 1-inch mounds onto the prepared baking sheets.

Bake for 105 minutes until the meringues are dry to the touch and peel away easily from the parchment.

Turn off the oven and leave the meringues inside with the door slightly ajar for 60 minutes to cool completely and prevent cracking.