Preheat the oven to 400°F (200°C) and line a baking sheet with parchment paper. Make sure the oven is fully preheated before the poppers go in so the jalapeños roast properly and the cheese melts evenly.

Wash and thoroughly dry the jalapeños. Slice each one lengthwise, keeping the stem attached if desired. Use a small spoon to carefully remove the seeds and white inner membranes. Remove all of the membrane for a milder appetizer or leave some behind if you prefer more heat.

Add the softened cream cheese, Brie cheese, garlic powder, fresh thyme, black pepper, and salt to a mixing bowl. Stir until the ingredients are well combined. The mixture does not need to be perfectly smooth; small pieces of Brie will melt into delicious creamy pockets while the poppers bake.

Gently fold the fresh blueberries into the cheese mixture. Try not to crush the berries because keeping some of them whole creates small bursts of blueberry flavor throughout the filling. If your blueberries are unusually large, cut them in half before adding them.

Spoon the blueberry and Brie filling evenly into each jalapeño half. Fill the peppers generously but avoid creating a large mound above the edges because the cheese will spread as it melts.

Arrange the filled jalapeño halves on the parchment-lined baking sheet in a single layer. Leave a little space between each pepper so hot air can circulate around them and the jalapeños roast rather than steam.

Bake for 15–20 minutes, or until the jalapeños are tender and the cheese filling is hot, melted, and lightly browned around some of the edges. Start checking around the 15-minute mark because smaller peppers may finish sooner.

If you want a more deeply golden top, switch the oven to broil for the final 1–2 minutes. Stay close to the oven and watch carefully because the cheese can brown very quickly under the broiler.

Remove the poppers from the oven and allow them to rest for 3–5 minutes. This gives the melted cheese a chance to settle slightly and makes the poppers easier to serve without the filling immediately spilling out.

Garnish with a little additional fresh thyme or freshly cracked black pepper if desired. Serve warm as an appetizer, party snack, BBQ side, or keto-friendly finger food.