Pre-heat oven to 350ºF (180ºC). Grease a 9-inch bundt pan generously with oil and dust with flour. This will prevent the cake from sticking.
To a medium mixing bowl, mix together buttermilk, eggs, oil, sugar, lemon zest, and vanilla extract (if using).
To a small bowl, mix the flour with baking soda and cinnamon (if using).
Add dry ingredients into the wet and mix together just until combined. Don't overmix! Stir in the blueberries.
Pour the mixture into the greased pan and bake for 45-50 minutes, until a toothpick comes out clean. If the cake is getting very brown on the top and is still not cooked at the bottom, cover it with aluminum foil. I find that in the last 10 minutes I always have to do that.
When ready, take the cake out of the oven and let it cool in the pan for exactly 10 minutes (set a timer). Use a table knife to gently push the cake away from the sides of the pan. Then place a wire cooling rack (or a plate) over the base of the cake and invert the pan to release. Let it cool completely before icing and serving.