Wash the blackberries and drain. In a small saucepan (no lid), cook blackberries in water for 5 minutes. Meanwhile combine the sugar, cornstarch, and salt in a large custard cup.
Add sugar mixture to blackberries and stir until thickened and translucent. Cook another 5 minutes, stirring occasionally to keep the mixture from sticking.
Remove from heat and add lemon juice. Let sit for 5 minutes.
Serve with thick cream to swirl in.
Beverly Pepper, The Glamour Magazine After Five Cookbook, 1952 (modified)http://macheads2.net/recipes/BlackberryFlummery.html