Using a blender, blend the milk, cream, salt, sugar, and blackberries. Taste and add additional salt or sugar as needed; the sweetness of the berries (and your palate) will vary. Strain the mixture through a sieve to remove any seeds. Churn the mixture in an ice cream maker using the unit’s instructions. Mine takes about 20-25 minutes to reach soft serve consistency in a Cuisinart frozen canister maker. Transfer to an airtight container and freeze until hard.