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In a medium bowl, muddled the blackberries (smash 'em really good), and then sprinkle with sugar. Set aside.
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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.
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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
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Add the blackberry juice, lime juice (about 1 tablespoon), and tequila to to the bowl and whisk to combine.
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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.
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Top each shot cup with a plastic lid. Right before serving, sprinkle the top of the shot with lime zest, if desired.