Using small sharp knife, remove peel and white pith from grapefruits. Working over bowl to catch juice, cut between membranes to release segments. Reserve juice in bowl. Arrange grapefruit segments decoratively on 4 plates, dividing equally.
Combine sugar, water and reserved grapefruit juice in heavy small saucepan. Stir over low heat until sugar dissolves. Remove saucepan from heat. Stir in crème de cassis. Drizzle syrup over grapefruit. Refrigerate at least 2 hours or up to 6 hours. Garnish with mint, if desired, and serve.