1 tablespoon white sugar
1 tablespoon lemon juice
1 cup ice cubes, or as needed
2 fluid ounces vodka
2 fluid ounces black cherry juice
1/2 fluid ounce grapefruit juice
1 teaspoon agave nectar
1 maraschino cherry
Spread sugar over a small plate. Pour lemon juice onto another small plate. Dip the rim of a martini glass in the lemon juice, then into the sugar to coat.
Fill cocktail shaker with ice. Add vodka, black cherry juice, grapefruit juice, and agave nectar. Cover shaker with lid and shake vigorously until chilled, about 10 seconds. Strain drink into prepared martini glass and garnish with maraschino cherry.