Preheat the oven to 350F and prepare 2, 8" round cake pans.
Grate a large zucchini and place in a clean dishtowel. Roll up the towel and wring until most of the liquid comes out then set aside.
In a large bowl, using a hand mixer, beat together the eggs, oil, vanilla, and sugar.
Fold in the zucchini.
Slowly add in the flour, baking powder, and baking soda.
Gently fold in the blueberries.
Divide batter evenly between the two cake pans, lightly tapping on the counter a few times to remove any air bubbles.
Bake for 35-40 minutes or until a toothpick inserted in the center comes out clean and the top springs back to the touch.
Cool for 20 minutes in the pan, then turn out onto a wire rack and cool completely.
Make the Buttercream
While the cakes are cooling combine the butter and salt in a large bowl or stand mixer and beat well until creamy.
At slow speed, add in powdered sugar ½ cup at a time until smooth.
Add the lemon juice, zest, vanilla and food coloring (if using) and beat on medium-high for another 3-5 minutes until light and creamy.
Frost the cake any way you like - simple... fancy... it's all good.
Preparing the pans. I choose to use the cooking spray that has added flour. You can used the tried and true method of greasing and flouring the pan. You can also go the parchment paper route. I've never had a problem at all with the cooking spay (unless you're using a silicone pan). it really is a personal preference.
Some people dust the blueberries with flour before adding to keep them from sinking. I usually don't with this recipe because it's a rather sturdy batter but it certainly won't hurt anything. As an FYI there are about 2 cups of blueberries in a pint. If you have a little more - fine. If you're short a little bit (or save some for garnish) that's ok too.
For a bigger blueberry punch add a thin layer of blueberry preserves in the middle layer before you add the buttercream.
Food coloring. To use or not to use? That is completely a personal preference. With out it the buttercream will be an extremely light pale yellow. It's still beautiful and if the cake is garnished with a couple of thin lemon slice curls everyone will know to expect the fresh lemony taste. A few drops of food coloring simply boosts the color. If you decide to use it, put in one drop at a time until it's thoroughly mixed in and decide what looks best to you. I use 2-3 drops at most.
Because of the butter, as well as the fruit and vegetable content, store this cake in the refrigerator and bring it out just 5 or 10 minutes before serving.