Preheat your oven to 200°F (93°C). Line a baking sheet with parchment paper.
In a clean mixing bowl, beat the egg whites using an electric mixer on medium speed until foamy.
Gradually add granulated sugar, one spoonful at a time, while increasing the mixer speed to high. Continue to beat until stiff peaks form and the mixture is glossy.
Add the vanilla extract and white wine vinegar, mixing gently until incorporated.
Carefully fold in the blackberry puree using a rubber spatula, creating a marbled effect in the meringue.
Using a spoon or a piping bag, drop or pipe small dollops of meringue onto the prepared baking sheet, leaving space between each one.
Bake in the preheated oven for about 2 hours, or until the meringues are dry and can easily peel off the parchment paper.
Turn off the oven, leaving the meringues inside to cool completely for another hour.
Once cooled, garnish with fresh blackberries before serving.