Preheat the oven to 350 degrees F and grease a 9 inch square or round pan.
In a medium bowl, whisk together the flour, baking powder and salt. Set aside.
In a large bowl with an electric hand mixer (or stand mixer with the paddle attachment), cream together the softened vegan butter, sugar and lemon zest until fluffy and creamy.
Beat in the vanilla and vegan yogurt or applesauce. Alternate adding the flour mixture and soy milk, in 2-3 parts and mix on low speed until incorporated. Scrape down the sides and bottom of the bowl as needed. Be careful not to over mix the batter.
Toss the blueberries in a bowl with the tablespoon of flour, then carefully fold into the cake batter, being careful not to break the berries (as much as possible).
Add the cake batter to the pan and spread evenly; it will be thick.
In a medium bowl, combine the flour and sugars for the streusel topping. Add the cinnamon, if using. Drizzle the melted vegan butter into the bowl and stir with a fork until crumbs form.
Sprinkle the streusel topping over the cake batter and bake for 45-55 minutes, until a toothpick inserted in the middle comes out clean.
Transfer to a wire rack and let cool for 30 minutes before serving.