Mix the granulated sugar, brown sugar, salt, and lemon juice in a medium to large mixing bowl large enough to hold all of the pie filling.
⅓ cup (67 grams) granulated sugar, ⅓ cup (64 grams) brown sugar, 2 tablespoons lemon juice, ⅛ teaspoon salt
Core, peel, slice the apples, and toss them with the sugar and lemon juice mixture to prevent browning.
4 medium sized apples
When you have cut all of the apples, give them a final toss with the sugar and lemon juice, and place them in a sieve over a medium sized pot to drain for at least 30 minutes. Don't wash the apple bowl yet.
Make the blackberry puree by blitzing the blackberries in your food processor or blender. Pour them into a sieve to strain out the seeds and other solids.
12 ounces (about 3 to 3.5 cups) blackberries, fresh or frozen
Combine the blackberry puree with the juice that has drained from the apples and add the cornstarch. Heat this liquid over low heat, stirring occasionally, until the mixture has thickened.
2 tablespoons cornstarch
Meanwhile add the apples back to the large bowl that previously held the apples.
When the blackberry and apple juice mixture has reduced to a thick syrup, add the vanilla, then add this liquid to the bowl with apples and cornstarch. Toss gently and let cool a few minutes.
1 teaspoon vanilla