Combine blackberries, blueberries, sugar, lime juice & zest and cornstarch in slow cooker.
In a medium bowl, combine flour, sugar, baking powder and salt. Using your hands, knead in cold butter until coarse. Add milk and vanilla and stir until combined. Spoon flour mixture over berries.
Cover and cook on low for four hours, or until mixture is set and berries are bubbling.
