Combine the white rum, lime juice, grapefruit juice, maraschino liqueur, and crushed ice in a blender.
Blend until the mixture is smooth and the ice is finely crushed.
On a small plate, mix a pinch of lime zest with a teaspoon of granulated sugar for the glass rim.
Moisten the rim of a chilled glass with a lime wedge, then dip it into the lime-sugar mixture to coat.
Pour the blended cocktail mixture into the prepared glass.
Garnish with a lime wheel, if desired, and serve immediately.