Pour black sanding sugar onto a saucer, making sure the layer is at least ΒΌ inch high and wider than the rim of your goblet or cocktail glass.
Dab the rim of your goblet with simple syrup (or another sticky liquid). Then, dip the coated rim into the black sanding sugar on the saucer, gently twisting the goblet to ensure the entire rim is evenly covered with sugar.
In a cocktail shaker filled halfway with ice, combine the dark rum, pineapple juice, fresh lemon juice, and Blue Curacao syrup. Shake well to chill and mix the ingredients.
Carefully strain the cocktail into your prepared goblet, taking care not to splash the sugar-coated rim.
Top off the cocktail with lemon-lime soda, adding a fizzy finish.
Serve immediately and enjoy your Green Goblin Cocktail!