1 cup blackberries or black currants
½ cup sugar
1 Tbsp cassis
1 tsp lemon juice
Simmer the plums with the blackberries, sugar, and about 1⁄4 cup water (more or less depending on the ripeness of the fruit) in a small saucepan over low heat until soft and pulpy, 5 to 10 minutes. Let cool slightly, then puree and press through a sieve to remove seeds and skin. You should have about 2 cups puree. Stir in the cassis and lemon juice to taste. Chill thoroughly, then churn in an ice cream maker. Freeze until hardened, about 1 hour.
Recipe and photo from PlumGorgeous: Recipes and Memories from the Orchard by Romney Steele/Andrews McMeel Publishing, 2011.