Assemble 4 double-decker peanut butter and jelly sandwiches, with a slice of bread separating the peanut butter and jelly (each sandwich will use 3 slices of bread). Using a serrated knife, remove the crusts from the sandwiches and discard. Gently slice each sandwich into six 1-inch squares.
Gently slide 1 cube of sandwich onto an 8-inch wooden skewer. Place 1 slice of banana and 1 blackberry above the sandwich. Repeat this process three more times, until 4 cubes of sandwich, 4 banana slices, and 4 blackberries are used, being sure to cover the pointed end of the skewer with a blackberry.
Assemble 5 more skewers and place on a serving platter.