Add 1 cup sugar and the water to a small saucepan on medium heat.
While the syrup is cooking, add the egg whites and cream of tartar to the bowl of a stand mixer fitted with the whisk attachment and beat on high speed. When the egg whites start foaming, slowly add the remaining sugar and continue beating until stiff peaks form. If your syrup is not ready, lower the speed, but do not turn the mixer off.
The syrup is ready when it pours off the spoon in a steady stream or reaches 235F.
With the stand mixer on high speed, slowly pour the syrup into the egg whites, but DO NOT let the syrup hit the whisk. It should be a steady stream into the bowl.
Continue beating the frosting until the bowl cools down and feels barely warm. Switch to the paddle attachment and add the butter one tablespoon at a time.
Continue beating until frosting stiffens. If it’s not stiffening then stick the bowl in the fridge for 20 minutes, and then try again.
Put the blackberries in a blender and blend until there aren’t any blackberry chunks left. Pour blackberry mixture into a mesh strainer that’s over top a bowl to separate the seeds. Using a spoon stir and push the mixture against the strainer to get as much in the bowl as possible. There should be about ½ cup to use in the frosting.
Add the seedless blackberry juices and the vanilla in the frosting and beat until well combined.