Place a metal mixing bowl in the refrigerator to chill for making whipped cream.
Combine the blackberries, lemon juice, and sugar in a heavy-bottomed saucepan and bring to a simmer. Simmer until the berries break down, thicken slightly, and become syrupy (about 10 minutes).
If desired, strain the berry syrup through a fine mesh sieve. Discard seeds, add two teaspoons of vanilla and set the syrup aside to cool.
Combine the whipping cream, maple syrup, and remaining teaspoon of vanilla in the chilled bowl.
Using an electric mixer, beat cream on low speed until it starts to thicken. Increase the speed to medium-high and beat until stiff, firm peaks form.
Stop whipping before the whipped cream begins to clump, and store it in the refrigerator until the blackberry syrup is cool.
Put just enough cooled blackberry syrup in six individual dessert dishes to cover the bottom.
Gently add the remaining blackberry syrup into the chilled whipped cream and fold the two together 5-6 times, until the syrup is swirled through the whipped cream.
Carefully scoop even amounts of the blackberry fool into the dishes with the syrup on the bottom. Top with fresh berries and serve.