Cut the peaches into slices. Reserve 8 slices for the sundaes. Cut the remaining slices into cubes. Reserve 12 blackberries for the sundaes.
In a medium-sized saucepan, combine the sugar and water. Heat to boiling over medium heat. Stir in the peach cubes and remaining blackberries. Continue simmering, stirring frequently, until peaches and berries can easily be mashed with a spoon, about 10 minutes. Stir in cinnamon.If desired, mash some of the fruit with the mixing spoon while stirring for a less chunky sauce.
Stir together cornstarch and 1 T. water until smooth. Slowly pour into the simmering sauce and stir well. Continue to simmer the sauce until thickened to the consistency of a thick syrup. Remove from heat and let cool at least 5 minutes before spooning onto ice cream.
Scoop ice cream into 4 serving dishes. Spoon on the berry syrup and top each sundae with 2 peach slices and 3 blackberries. Enjoy!
Notes
Refrigerate any leftover sauce and use for more sundaes or as a breakfast syrup.