Preheat oven to 400°F (200°C) and line a baking sheet with parchment paper.
Slice jalapeños in half lengthwise and use a small spoon to scrape out all seeds and white membranes. Wear gloves if you have sensitive skin.
Combine brie cubes with blueberries, honey, chopped thyme, black pepper, and salt in a mixing bowl. Mix gently to avoid crushing berries completely.
Spoon the cheese mixture generously into each jalapeño half, packing well but not overfilling.
Wrap each stuffed pepper with prosciutto or bacon if using, and secure with a toothpick.
Arrange poppers on baking sheet and brush lightly with olive oil.
Bake for 12-15 minutes until cheese is bubbly, peppers are tender, and any bacon or prosciutto is crispy.
Let cool for 3-4 minutes, then drizzle with warm blueberry preserves if using, garnish with fresh thyme, and serve warm.