In a medium bowl, muddled the blackberries (smash 'em really good), and then sprinkle with sugar. Set aside.

Place gelatin in a medium mixing bowl. Pour cold water over the gelatin and stir to combine. Let stand for 1 to 2 minutes until the gelatin has absorbed the water. Pour in the boiling water, and whisk until the jello is dissolved.

Mash the blackberries and sugar together one more time, and then run them through a mesh sieve so you're left with the juice. Discard the pulp

Add the blackberry juice, lime juice (about 1 tablespoon), and tequila to to the bowl and whisk to combine.

Pout the jello mixture evenly into each of the plastic cups. Place the cups in the refrigerator and chill until set, at least 1 hour.

Top each shot cup with a plastic lid. Right before serving, sprinkle the top of the shot with lime zest, if desired.