Preheat your oven to 200°F (95°C) and line a baking sheet with parchment paper.

In a clean mixing bowl, combine the egg whites and cream of tartar. Using an electric mixer, beat on medium speed until soft peaks form.

Gradually add the granulated sugar, about a tablespoon at a time, while continuing to beat the egg whites. Continue beating on high speed until stiff peaks form and the mixture is glossy.

Add the vanilla extract and a pinch of salt, mixing just until combined.

Gently fold in the blackberry puree with a rubber spatula, being careful not to deflate the meringue too much.

Transfer the meringue mixture into a piping bag, if using, and pipe small mounds onto the prepared baking sheet. If you don't have a piping bag, you can also spoon the mixture onto the sheet.

Bake in the preheated oven for 90 minutes, or until the meringues are dry to the touch and can easily be lifted from the parchment paper.

Once baked, turn off the oven and let the meringues cool inside with the door slightly ajar for about 1 hour.

Serve as is or garnish with additional blackberry puree if desired.