In 2-quart saucepan, beat cold water and cornstarch with whisk until cornstarch is dissolved. Add the 2 1/4 cups blackberries, the sugar, maple syrup and vanilla. Heat to boiling over medium-high heat, stirring frequently and breaking up blackberries with a fork until mashed. Reduce heat; simmer 5 to 10 minutes. Remove from heat; set aside. The mixture will thicken slightly as it cools.