Place blackberries in blender and puree until smooth. Strain through sieve to remove seeds. You may need to press the juice through with the back of a spoon.
Rinse out blender, and place yogurt, banana, mango, milk and sugar (if using) in blender. Blend together until smooth. If desired, add ice to blender and blend to thicken the smoothie more.
Layer smoothie by placing ⅓ of the mango mixture in glass, then top with tablespoon or more of blackberry puree. Continue to layer until glass is full.
Top with fresh blackberries and garnish with mint sprig if desired.