Place the sugar and cassis in a large (8-by-4-inch) heavy-bottomed saucepan. Add the rhubarb, blueberries, apple, and lemon zest and toss well to coat the fruit with the cassis and sugar. Cover the pot and cook over medium heat for 5 to 10 minutes, until there is a lot of liquid and it comes to a full boil. Lower the heat and cook uncovered at a low boil for 35 to 40 minutes, stirring occasionally, until the fruit has disintegrated and the mixture has thickened. (I test a little by putting some jam in a dish in the freezer to check the consistency when it cools.) If necessary, cook for another 5 minutes. Set aside to cool, then refrigerate.