Place 1 punnet of blueberries into a glass bowl and crush berries with a fork. Pour over vinegar, cover and set aside overnight. Drain into a small saucepan and discard berry pulp. Add sugar and stir over medium heat for 5 minutes until sugar has dissolved. Simmer for 10 minutes, skimming off any froth that comes to the surface. Pour into a clean jar and cool completely.
Heat a frying pan over medium-high heat. Add walnuts and pepitas and stir constantly for 2 minutes until browned and crisp. Transfer to a bowl.
Shave cucumbers into ribbons using a vegetable peeler. Place salad greens into a bowl. Add cucumber, onion, remaining blueberries and crumbled gorgonzola.
Whisk 1/4 cup of the blueberry vinegar with oil and mustard until creamy. Season to taste. Drizzle over salad and serve.