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Preheat the oven to 350°F with a rack in the center. Grease an 8 1/2" x 4 1/2" loaf pan and line with a parchment paper sling.
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To make the blueberry jam: In a small saucepan, combine all of the jam ingredients and place over medium-low heat. Cook, stirring often and mashing the blueberries with a spatula, until the blueberries are mostly broken down and the mixture starts to thicken, about 10 to 12 minutes. (The jam is ready when you drag a spatula across the bottom of the pot and it leaves a line for about 1 second before filling in.) Transfer the jam to a bowl and set aside to cool while you prepare the batter.
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To make the batter: In a medium bowl, whisk together 2 cups (240g) of the flour, baking powder, and salt.
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In a large bowl or the bowl of a stand mixer, add the sugar and lemon zest. Rub the sugar and zest together with your fingertips until the mixture is fragrant and resembles damp sand.
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Using an electric hand mixer or the flat beater attachment of the stand mixer, add the eggs and mix to combine. Add the sour cream, lemon juice, and vanilla and mix to combine. Add the cooled melted butter and mix to combine.
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Add the dry ingredients in two additions, mixing after each until just combined. Use a flexible spatula to scrape the bowl to ensure the batter is evenly mixed; this is the lemon batter.
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In a medium bowl, combine 1/3 cup (about 100g) of the blueberry jam and the remaining 1 tablespoon flour, and stir until well combined. Add about 1 cup (225g) of the lemon batter to the bowl and stir until evenly combined; this is the blueberry batter.
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Using a scone and muffin scoop or large spoon, portion about one-third of the lemon batter into the bottom of the prepared loaf pan. Use the back of the spoon or a small spatula to spread the batter into an even layer. Dollop about half of the blueberry batter on top of the lemon batter. Spread it into an even layer.
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Repeat with another one-third of the lemon batter, followed by the remaining blueberry batter, and then the remaining lemon batter, spreading each portion into an even layer before adding the next.
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Use a small offset spatula or skewer to gently swirl the two batters together in the pan by making several figure-eight motions.
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Bake the lemon blueberry loaf for 55 minutes to 65 minutes, tenting the loaf with foil near the end of baking, if necessary, to prevent overbrowning. The loaf is done when a toothpick or paring knife inserted into the center comes out clean or with a few moist crumbs attached.
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Use the parchment sling to lift the lemon blueberry loaf out of the pan and transfer it to a wire rack; remove the parchment sling immediately. Let the loaf cool completely, at least 1 hour.
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To make the glaze: In a medium bowl, add the confectioners’ sugar and jam. Whisk until a thick, smooth glaze forms. Add water by the 1/4 teaspoon to make a pourable glaze.
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Pour the glaze over the cooled loaf, concentrating it in the center and letting it drip down the sides. Let the glaze set for 30 minutes before slicing and serving.
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Storage information: Store leftover lemon blueberry loaf in an airtight container at room temperature for up to 3 days.
