1. Combine the black raspberries and sugar in a heavy bottomed, non-reactive pot. Bring to a boil, stirring occasionally over medium-high heat. Don't worry, the heat will cause the berries to release their juice, which will keep the sugar from burning. Adjust the heat so that the mixture simmers. Cook until the mixture is very liquid and the remaining berries are soft, about 10 minutes.
2. Strain the berry mixture through a fine mesh or cheesecloth. Use a rubber spatula to push as much juice through as possible. Discard the seeds and return the liquid to the pot.
3. Bring the liquid back to a boil and add the lemon juice and Grand Marnier. Store the sauce in jars in the refrigerator for up to 3 weeks or freeze for longer storage.Serving Size: about 1 tablespoon