Fill a rocks glass with crushed ice, try to feather the ice in lightly to avoid it compacting. Add the first 3 ingredients with cubed ice to a cocktail shaker, shake vigorously, and fine strain into the rocks glass. Feather some more crushed ice on top if necessary. Lightly drizzle the Crème de Mure on the top of the drink so that it trickles down through the crushed ice. Garnish with a lemon wheel and blackberry.