Prepare the gelatin mix according to the package instructions, using the seltzer in place of the cold water. Allow to cool for 30 minutes.
Meanwhile, place a mandarin followed by a blackberry in the grooves of a 2.1-quart fluted bundt pan. Repeat to form a circle. Pour in just enough of the gelatin to cover the fruit (about 1 1/2 cups). Chill until set, about 1 1/2 hours, leaving the remainder of the gelatin mix on the counter.
Sprinkle the blueberries in an even layer over the firm gelatin. Cover with the remaining mandarin oranges. Top with the remaining blackberries. Pour the remaining gelatin mixture over the fruit. Chill until firm, at least 8 hours and up to overnight.
When ready to serve, place the gelatin mold in a bowl of warm water for 2 minutes to loosen. Invert onto a circular serving platter. Fill the center of the mold with fresh berries to serve.