Combine mango, cold water, sugar substitute, corn starch, lemon juice and cinnamon in a medium sized pot and stir to coat evenly. Let sit for 10 minutes. Meanwhile prepare biscuit topping.
Preheat oven to 400 degrees F.
In a medium bowl combine the flour, sugar substitute, baking powder, baking soda, cinnamon, and salt. Whisk to combine.
In a small bowl beat together the greek yogurt, egg and melted butter. Pour the wet ingredients into the dry ingredients and mix to form a dough.
Cook the mango mixture over medium heat until thickened and bubbly. Add the blackberries and gently stir until heated through. Pour mixture into a 2 quart casserole dish and drop the biscuit dough topping in small mounds onto the hot filling.
Bake for 20 minutes until browned and a toothpick inserted comes out clean.