Black Magic Jello-O Shots

In a large measuring cup, whisk together 1 package purple Jell-O and 1 cup boiling water until completely dissolved. Whisk in vodka. Fill shot cups halfway with mixture. Transfer to refrigerator to firm up, 2 to 3 hours. When shots are almost firm, make the second layer: In a large measuring cup, whisk together remaining package purple Jell-O and 1 cup boiling water until completely dissolved. Whisk in vodka and black food coloring.  Pour black Jell-O mixture over each Jell-O shot and return to refrigerator to firm up, 2 to 3 hours more. Once Jell-O shot is firm, rim each shot glass with lime and dip in black sanding sugar.