In a large, heat-proof bowl, carefully pour 4 cups of boiling water over the cabbage. Allow to sit for 5 minutes, or until the water is very saturated with blue or blueish purple color.
Strain the mixture to remove the cabbage and pour the blue water into a large container such as a 13×9″ baking dish. Mix in 1 1/2 cups of tequila. Cover and freeze.
In another container, combine the limeade, triple sec, 4 cups of water, and 1 1/2 cups of tequila. Cover and freeze until solid.
Use a spoon or ice cream scooper to stir and break apart the ice to create a slushy consistency. Scoop the margarita mix into glasses just over 3/4 of the way full, then top with the blue slushy ice. Stir together for the color-changing effect and enjoy!