Bring blackberries and water to a boil in a 3-quart saucepan. Reduce heat, and simmer 5 minutes or until blackberries are soft.
Mash blackberries with a potato masher or fork; pour through a large wire-mesh strainer into a bowl, using the back of a spoon to squeeze out the juice. Discard the pulp and seeds.
When fresh blackberries aren't available, substitute 2 (16-ounce) packages of frozen blackberries, thawed.