Toss the blackberries into your blender, along with the cucumbers, 2 tbsp of water and fresh squeezed lemon juice. Blend until completely blended, about 30 seconds.
Place a wire meshed strainer over a bowl and pour the blackberry cucumber mixture onto the strainer. Using a spatula or the back of a spoon, press the mixture into the strainer until all of the juice fills up in the bowl below.
Pour the juice into a cocktail shaker. Now you are going to add the gin. You can use as much or as little as you desire, but for me, I used 2 shots, or 3 fluid ounces, to keep it light. Drop in a few ice cubes, and shake well.
Divide the mixture evenly into 3 glasses (should be about 1/3 cup of liquid per glass), and top with equal amounts of tonic water! Garnish with cucumber slices, black berries and/or mint! Enjoy!