Ensure the butter, eggs, sour cream, and lemon are at room temperature before beginning. Preheat the oven to 350°F.
In a stand mixer with the paddle attachment, hand mixer, or with some elbow grease, beat the butter until light and fluffy, about 5 minutes. Add in the sugar and cream together for another 5 minutes. Add the sour cream, oil, and eggs one at a time. Mix slowly to incorporate. Add the lemon juice, zest, vanilla and lemon extracts. Mix well.
Add the gluten-free flour, baking powder, baking soda, and salt. Mix until just combined.
Toss the fresh blueberries with 2 tablespoon of gluten-free flour. Pour into the batter and gently fold with a rubber spatula until the blueberries are evenly dispersed.
Pour the mixture into a greased 9x5" loaf pan. I used a silicone loaf pan inserted into a metal one (to keep its shape). Bake on the middle rack in the oven for approximately 65 - 70 minutes. Check that a skewer inserted comes out clean. If so, it's done. Cool for 10 minutes in the pan, then remove to a wire rack and allow to cool completely (at least 2 hours).
While the loaf is cooling, you can prepare the icing glaze. Pour about 1 teaspoon or lemon juice and 1 teaspoon of milk into the icing sugar. Mix with a spoon until smooth. Add more liquid if a thinner consistency is desired. I like mine thick but pourable.
Once the loaf is completely cool, pour the glaze on top and spread with an offset spatula. Allow the glaze to solidify for a few minutes. Slice and enjoy! See serving and storage instructions below.