For the tart crust: In a large bowl combine the flour and salt. Cut the chilled butter into 1-inh pieces. Using a pastry cutter or a fork, cut the butter into the flour. Cut together until the mixture resembles coarse crumbs. Add the ice water 1 tablespoon at a time, and mix very gently, just until the mixture can be formed into a ball. Do not overwork the dough. Press the ball into a flat disk. Wrap in plastic wrap and refrigerate for 30 minutes. Preheat the oven to 350 degrees. Roll the dough out to 1/8 inch thick and place into a tart pan. Trim the edges. Cover the crust loosely with parchment paper and weight with pie weights or dried beans. Blind bake the pie crust for 15 to 20 minutes, or until golden brown. Let cool completely.
For the Fool: In a saucepan, combine the rhubarb, sugar and lemon juice. Cook over medium heat until the rhubarb is very soft, about 15 minutes. Remove the mixture from the heat. Puree the mixture in a food processor. Refrigerate until cold. Fold the puree into the whipped cream. Refrigerate for several hours. 1 hour before serving, spoon the fool into the prepared tart crust. Place the blackberries on top of the tart.