Set a large rectangular wooden board (at least 35 x 50 cm) on a flat surface.
Arrange the purple grapes in a wide, slightly curved row across the bottom third of the board to form the brim of the hat.
Press the chocolate segments in a straight horizontal line just above the purple grapes to form the hat band, spanning roughly 30 cm.
Pile the green grapes above the chocolate band in a widening triangle, angling the sides inward as you work upward.
Cover the upper two-thirds of the triangle with blackberries, pressing them gently into the grapes so they sit flat.
Use a small cluster of 8 to 10 extra blackberries to form the curled tip of the hat, setting them at a slight diagonal above the main triangle.
Refrigerate uncovered for up to 4 hours, or serve immediately.