For the syrup, in a medium saucepan* combine 2 cups of the berries, the sugar, water, and a dash of salt. Bring to boiling; reduce heat and simmer 10 minutes stirring occasionally. Remove from heat; cool slightly. Strain through a colander, pressing gently without crushing the fruit; discard berries. Cool syrup.
For sangria, in a gallon jar or pitcher combine the blackberry syrup, rosé wine, cognac, lemon juice, fruit, and a few sprigs of basil, if desired. Cover and refrigerate 4 to 24 hours.
Serve with ice, additional fruit, and basil. Makes 13 cups (26, four-oz. servings).
*Test Kitchen Tip: Use a nonreactive saucepan, such as Teflon-lined, enamel, stainless steel, or silver. The acid in the berries may react with metal.