Preheat oven to 350 degrees F. Grease an 8 1/2 by 4 1/2-inch or 9 by 5-inch loaf pan then dust with flour, shake out excess and set aside.: As the oven warms you will smell a faint dry heat and the kitchen will begin to feel cozy, which mentally prepares you for baking. Greasing and flouring the pan gives the loaf a reliable release and an even outer crust, you want a thin, even coating so the bread does not stick. A frequent issue is over greasing which can create a greasy edge, or under greasing which causes tearing when you invert; make sure to tap out the excess flour evenly. Visually, the pan should have a soft sheen with no clumps of oil, and the flour dust should be uniform. Using the right pan size matters, as an undersized pan will cause overflow, and an oversized pan will make a flatter loaf.
In a medium mixing bowl whisk together flour, baking powder and salt.: When you whisk these dry ingredients together you are integrating leavening agents so the lift is even throughout the loaf. The sound is a soft swish, and the visual cue is a uniform pale mixture with no streaks of baking powder or salt. This blending keeps pockets from forming that can create odd dense areas. A common misstep is failing to sift or whisk which can lead to clumps in the batter, so be thorough until the mixture looks homogeneous. Properly combining ensures predictable rise and texture.
Rinse blueberries and drain very well, transfer to a medium bowl and add 1 tbsp of the flour mixture and toss to coat, set aside.: The berries should be cool and dry, not dripping, because excess moisture will change batter consistency and baking time. Tossing them in that tablespoon of flour prevents sinking by giving them a light coating, which helps them suspend in the batter during baking. The gentle clinking of berries in a bowl is a small, satisfying sound; visually you will see a faint dusting of flour on each berry. Avoid over rinsing or leaving them wet, or their juices will bleed into the batter and color it. If your berries are particularly small or delicate, handle them with a light touch to prevent crushing.
In the bowl of an electric stand mixer fitted with the paddle attachment, whip together sugar, lemon zest and butter until mixture is pale and fluffy.: As you beat the unsalted butter and sugar, you will notice it lighten in color and texture, becoming airy and slightly glossy; this trapped air contributes to lift. The citrus oils from the zest will perfume the mixture, giving an immediate lemony aroma that wakes the senses. If your butter is too cold it will not cream properly and the mixture will remain lumpy, while too soft or melted butter will produce a loose batter. The correct texture looks fluffy and holds gentle peaks when lifted, and if it seems greasy, chill briefly and rebeat to incorporate air.
Mix in eggs one at a time and blend in vanilla.: Adding eggs slowly helps maintain an emulsion and prevents the batter from splitting. After each addition you should see the mixture smooth and slightly glossy, with a satisfying hum from the mixer. Room temperature eggs integrate more easily; cold eggs can cause the batter to look curdled for a moment, but gentle mixing will bring it back together. If the batter looks separated after all eggs are added, pause and scrape down the bowl, then beat at low speed until smooth. The aroma will deepen into a richer cake like scent as the eggs incorporate.
Add 1/3 of the flour mixture and mix just until combined then mix in 1/2 the sour cream and lemon juice and mix just until combined. Repeat process once more.: This alternating method keeps the batter tender by limiting gluten development, you will notice the batter becoming thicker then loosening again when you add the sour cream and lemon juice . Work gently and stop mixing as soon as the streaks disappear, because overmixing creates toughness. The texture should be smooth and slightly thick, not rubbery; the scent will be a balanced citrus dairy note. A typical mistake is dumping all the flour in at once and overworking it, which yields a denser crumb, so slow and steady is key.
Mix in last 1/3 of the flour mixture then remove bowl from stand mixer and gently fold in blueberries.: After the final flour addition the batter should be cohesive and hold a ribbon when lifted. Removing the bowl and folding by hand preserves the airy texture and prevents the berries from breaking up. You want to fold with wide, gentle strokes so the blueberries stay intact and evenly dispersed; visually you should see small pockets of berry throughout without streaks of purple. A common error is vigorous stirring which can crush berries and color the batter, or overfolding which knocks out too much air, making the loaf heavy.
Pour batter into prepared loaf pan, spread even and bake until toothpick inserted into center comes out with a moist crumb or two, about 50 - 60 minutes.: As the batter hits the oven heat the aroma will intensify and a thin crust will form on top, often a pale golden hue. The correct doneness is a toothpick with a few moist crumbs clinging, not raw batter; you want that tender interior. Ovens vary, so start checking at 50 minutes and rotate the pan if your oven has hot spots. A mistake to watch for is opening the oven too often early on which causes the loaf to deflate, so resist peeking until close to the finish. Visual cues include a domed top and even golden color at the edges.
Let cool for about 5 minutes, run knife around edges to ensure loaf has loosened then invert onto a wire rack. Cool on rack about 30 minutes then finish cooling in an airtight container.: The initial rest lets the loaf settle and firms it slightly so it unmolds cleanly, you will notice a gentle steam release as you invert. Cooling on a rack prevents a soggy bottom by allowing air circulation, and an airtight container after cooling keeps the loaf moist. Avoid slicing too early or the crumb will be gummy and may tear; patience pays off. The loaf will continue to set as it cools, and the aroma will mellow into a comforting, stable scent.
Once cool, in a small mixing bowl whisk together powdered sugar and lemon juice (if you want it thicker add a little more powdered sugar), spoon over loaf.: The glaze should be silky and glossy, and when spooned on it settles into the loaf’s crevices adding a bright, sweet finish. Start with the listed ratio and adjust for thickness; the sound of the spoon lightly tapping the loaf is oddly satisfying. If the loaf is still warm the glaze may sink in rather than set on top, so ensure it is cool to the touch. A mistake is making the glaze too runny, which will pool at the base instead of creating that pretty sheen.
Let glaze set then cut into slices. Store in an airtight container.: Allow the glaze to firm so you get clean slices that look neat on a plate. When cutting, use a serrated knife with gentle sawing motions to preserve crumb and glaze. Stored properly, the loaf retains its texture for several days and the aroma will remain inviting. A common error is storing while slightly warm which traps steam and softens the crust; always cool completely before sealing.