Lassi is traditionally made with buttermilk, but this easy version uses a mixture of yogurt and water.
In a food processor, combine the blackberries, yogurt, water, sugar, and lemon juice, and puree. Taste and add more sugar or honey, as necessary. Sieve the mixture to remove the seeds, and reserve the liquid.
To serve, place 3 ice cubes into each of 2 serving glasses, pour over the lassi and garnish with a mint leaf.