Sauté onion in olive oil in a medium sauté pan until tender, about 5 minutes. Add mint, blueberries, sugar, vinegar and juice. Cook until juice reduces and sugar dissolves, 10 to 15 minutes. Let mixture cool to warm or room temperature.
To assemble: arrange baguette slices on platter. Top with a slice of brie. Spoon on blueberry compote. Serve at room temperature.