Preheat oven to 325 degrees. Lightly grease and flour, using gluten free flour blend, a 9X5 loaf pan and set aside.
In a large mixing bowl or stand mixer, cream together butter, sugar, lemon zest, lemon juice, vanilla, and almond extract until light and fluffy, about 3 minutes. Add sour cream and beat until incorporated. Add eggs, one at a time, beating on low speed after each addition.
In a smaller mixing bowl, sift together gluten free all-purpose flour, coconut flour, baking powder, and salt. Add dry ingredients to wet ingredients gradually while mixing on low speed. Beat for an additional 45 seconds, making sure to scrape down the sides as needed.
Mix batter well with spatula to make sure the bottom ingredients have been incorporated throughout. Pour into greased and floured loaf pan.
Bake at 325 degrees for 55-60 minutes or until top is golden brown in color and top springs back at the touch of a finger. Toothpick inserted into center should come out clean. Remove pan and cool on cooking rack for 20 minutes.
While cake is cooling, begin making simple syrup. Combine lemon juice and sugar in a small saucepan. Heat on medium heat until sugar is dissolved. Gently simmer for 2 minutes and remove from heat.
Once cake has cooled for 20 minutes, remove from pan. Poke numerous holes using a fork, toothpick, or cake tester. Brush syrup over the top of the cake and all around the sides. Allow to finish cooling.
While cake is continuing to cool, make vanilla glaze. In a small bowl combine powdered sugar, 1 tablespoon of milk, vanilla extract, and a pinch of salt. Mix together until a smooth glaze forms. Add more milk if needed to thin. Spoon on top of the cooled cake and allow to drip down the sides. Top with slivered almonds and blueberries if desired.