Preheat the oven to 375 degrees F. Grease a (9x5 inch) loaf pan.In a large mixing bowl, beat together the butter, 1/2 cup sugar, and lemon zest until combined. Add the sour cream and vanilla, beating until smooth. Beat in the eggs, one at a time, until combined. Add the flour, baking powder, and salt. Fold in 1 1/4 cups blueberries.Spoon half the batter into the prepared pan. Swirl in 1 tablespoon of blueberry jam, leaving a few streaks of jam throughout the batter. Add the remaining batter and then swirl in the remaining 1 tablespoon jam. Sprinkle on the remaining 1/4 cup blueberries and 1 tablespoon sugar. Transfer to the oven and bake for 55-60 minutes, or until center is just set. Let cool before glazing. Meanwhile, make the glaze. In a small bowl, whisk together the powdered sugar, and lemon juice until smooth and drizzly. Stir in the thyme, if using. Drizzle the glaze over the cake. Slice and enjoy!