Make the blackberry sauce: In a heavy saucepan melt the sugar over moderate heat, stirring with a fork, and cook the syrup, swirling the pan occasionally, until it is a golden caramel. Working carefully and quickly, stir in the water, the blackberries, the lemon juice, and the crème de cassis (the mixture will bubble up), stirring until mixture is combined well, and cook the mixture over moderately low heat until the caramel is dissolved. Strain the mixture through a fine sieve set over a bowl, pressing hard on the solids, and chill the sauce, covered, until it is cold.