How to Make It
Prepare the Pickled Blackberries and Blackberry Vinegar: Combine sugar, Champagne vinegar, water, tarragon sprigs, and peppercorns in a medium saucepan; bring to a boil over medium-high, stirring occasionally to dissolve sugar. Remove from heat, and let stand 15 minutes. Place blackberries in a large bowl. Pour vinegar mixture through a fine wire-mesh strainer over blackberries; discard tarragon and peppercorns. Cover blackberry mixture, and cool completely, about 30 minutes. Refrigerate blackberry mixture at least 2 days and up to 1 week. Transfer Pickled Blackberries to a separate container. Reserve Blackberry Vinegar.
Prepare the Shaved Cucumber Salad: Thinly slice cucumbers and onion using a mandoline. Place cucumber and onion slices, mint, and Pickled Blackberries in a medium bowl. Whisk together oil, Blackberry Vinegar, salt, and pepper; pour over cucumber mixture, and toss gently. Reserve remaining Blackberry Vinegar for another use. Refrigerate in an airtight container up to 1 month.