Place a handful or two of ice cubes in a tall high ball glass.
In a cocktail shaker, muddle the mint, blackberries, lemon slice, and sugar until the mint is very aromatic and slightly bruised looking, the sugar is mostly dissolved, and the blackberries and lemon have been flattened and released their juices.
Add the gin and shrub and top with ice.
Shake well, until the shaker is very cold and is sweating.
Strain into the prepared high ball glass, top with club soda, and garnish. You can garnish with a lemon wedge or slice, mint leaves, blackberries, and edible flowers if desired.