Add the vodka, sweet & sour mix, blue curacao, cranberry juice, and grenadine to a cocktail shaker. Add ice now if you are not going to add ice in the cup. Stir for a few seconds.
Place some light corn syrup on a small plate and on a separate small plate, place a small pile of purple sanding sugar.
Dip the rim of the glass in the syrup, pick up the glass and hold it over the plate and let the excess drip off.
Now dip the rim in the purple sugar until it is coated evenly all the way around.
Gently fill the cup with ice if desired. This will fill up the glass more and allow you to put a plastic spider on top, however this part can be skipped if you don't want to water down the cocktail.
Strain the cocktail into the glass.
Garnish with a maraschino cherry, a plastic spider, and a purple straw, if you'd like.