Sprinkle 2 envelopes gelatin evenly over vodka; let stand 3 minutes.
Combine 1 cup sugar, lemon juice, and 1/2 cup water in a medium saucepan over medium-high heat. Bring just to a boil, stirring until sugar dissolves.
Add hot lemon juice mixture to vodka mixture, stirring until gelatin dissolves.
Pour lemon-vodka mixture into an 8-inch square glass or ceramic baking dish coated with cooking spray; chill 1 hour and 45 minutes.
Place remaining 1/2 cup vodka in a medium bowl.
Sprinkle remaining 2 envelopes gelatin evenly over vodka; let stand 3 minutes.
Combine remaining 1/2 cup sugar, remaining 1/2 cup water, and 8 cups blackberries in a large saucepan; bring to a boil. Reduce heat; simmer 10 minutes, stirring mixture occasionally. Strain blackberry mixture through a fine sieve over a 4-cup glass measuring cup; discard solids.
Add 2 cups hot blackberry mixture to vodka mixture, stirring until gelatin dissolves (reserve remaining blackberry mixture for another use). Gradually pour blackberry-vodka mixture evenly over partially set lemon-vodka mixture. Chill 3 hours or until set.
Cut into 36 cubes.
Garnish with blackberries and lemon rind strips, if desired.
