Preheat oven to 350°F (180°C).
Brush a small 6 cup (18 cm) bundt pan with melted butter, then dust with flour.
Squeeze a lemon into a measuring jar, then add soy yogurt and dairy-free milk (it should be approx. 240 ml total amount of liquid). Mix to combine and set aside so it curdles and becomes vegan buttermilk.
Sift the flour and cornstarch into a large bowl. Add baking powder, baking soda and salt and mix everything together.
In another bowl, cream margarine with sugar, vanilla extract and lemon zest using an electric hand mixer. Stop the mixer to scrape the sides using a spatula to combine all the ingredients.
On low speed, mix in 1/2 of the flour mixture. Scrape down the sides of the bowl to incorporate. Add in the buttermilk and beat on low speed until incorporated followed by the remaining flour mixture (but please make sure not to overmix).
Gently fold the blueberries into the batter and pour it into the prepared pan. (Please read the information in the text above, if using frozen blueberries).
Bake for 50-60 minutes, or until a toothpick inserted into the middle of the cake comes out almost clean.
Allow to cool for 10-15 minutes in the pan, then invert it onto a wire cooling rack to cool further 5-10 minutes. Remove the pan carefully and let cool completely.