Macerate the berries. In a small bowl, combine the sugar and 3 blackberries. Let them sit at room temperature for about 5 minutes, allowing the berries to release their juices and become juicy.
Extract the juice. Press the macerated berries directly into a rock glass through a mesh strainer to extract their juices. Use the back of a spoon or a muddler to press out the juice thoroughly. Discard the remaining berry solids, or save them for another use (like a dessert topping).
Assemble the drink. Add ice to the rocks glass over the strained berry juice. Then, pour the bourbon, Angostura bitters, and water into the glass. Stir gently to mix the ingredients together, making sure that the berry juice is evenly distributed throughout the cocktail.
Garnish and serve. Garnish with an orange peel and a fresh blackberry on the rim of the glass. Optionally, let the cocktail sit for a milder drink for a few minutes before serving to allow the ice to melt slightly, diluting the drink and softening the flavors.