In a blender, add 200 grams of fresh blackberries and blend until smooth. If you prefer a smoother texture, you can strain the blackberry puree through a fine-mesh sieve to remove the seeds.
In the same blender, add 500 grams of chilled plain yogurt (dahi), 100 grams of sugar (adjust according to taste), 1/2 teaspoon of cardamom powder (elaichi), and the blackberry puree. Blend until well combined and smooth.
Add 100 ml of chilled water or milk to adjust the consistency of the lassi as per your preference. Blend again for a few seconds.
Pour the blackberry lassi into serving glasses. Garnish with a few fresh blackberries and mint leaves (optional). Serve chilled.