In a small saucepan combine 2 cups blackberries, 1 cup water, and sugar. Bring to boiling over medium-high heat, stirring to dissolve sugar. Reduce heat and simmer, uncovered, stirring occasionally, until berries soften, 5 to 7 minutes. Stir in honey to dissolve. Remove blackberry syrup from heat and press mixture through a fine mesh sieve. Add 1 cup water and lemon juice to strained mixture and stir until well combined.
In a large pitcher, combine blackberry lemonade, sparkling water, and gin. To serve, fill six glasses with ice and top with spiked lemonade. Garnish with 2 to 3 blackberries and a lemon slice.