Combine the blueberries, cucumbers, red onion, 1 tablespoon jalapeño, 1/4 cup cilantro, 2 tablespoons lime juice, 1 tablespoon olive oil, and the 1/4 teaspoon salt. Toss gently with a large spoon. Taste. Add more jalapeño, cilantro, lime juice, olive oil, and kosher salt to taste. Toss and taste again. Adjust as needed.
Set a large sauté pan or grill pan over high heat. Pat the fish dry with paper towels. Season with salt and pepper. Add a thin layer of oil to the pan. When it shimmers, carefully lower in the fillets. Reduce the heat to medium. Let the fish cook for two to three minutes undisturbed. Using a spatula, carefully nudge the fish to release it from the pan, then flip it. Cook for 1 to 2 minutes more, then transfer it to a serving plate. Spoon the blueberry salsa over top, and pass more on the side.