For best results, choose firm, plump blackberries so they hold their shape on the skewer, and use small, tender basil leaves to avoid overpowering the delicate sweetness of the fruit. If you prepare the skewers ahead, wait to add the oil, glaze, salt, and pepper until just before serving to keep the basil vibrant and the blackberries from becoming soggy. You can vary the flavor by using flavored balsamic (such as blackberry or fig) or add a tiny piece of prosciutto to each skewer for a sweet-salty contrast. Serve these chilled or at cool room temperature, and keep them loosely covered in the fridge if holding for more than 30 minutes.