1. Adjust the oven rack to the middle position and preheat the oven to 350 degrees Fahrenheit.
2. For the streusel topping, whisk together the flour, sugars, salt, and cinnamon in a mixing bowl. Rub the margarine or coconut oil into the flour mixture with your fingers until it reaches the consistency of wet sand. Add the pecans and toss with your fingers to combine.
3. For the cake, grease and flour a 9×13-inch pan (see note). In a large mixing bowl, whisk together the flour, salt, and baking powder. Using an electric mixer, beat the margarine or coconut oil, sugar, and lemon zest until light and fluffy, about five minutes. Add the eggs, one at a time, beating after each for a few seconds until incorporated. Stir in the flour mixture on the slowest speed to combine, alternating with the milk or creamer, beginning and ending with the flour. Stir in the blueberries and finish by scraping the bottom of the bowl and folding with a rubber spatula.
4. Scrape the batter into the prepared pan and smooth the top with a rubber spatula. Sprinkle the streusel topping evenly over the batter and bake for one and a half hours, rotating the pan halfway through baking, until a toothpick inserted between the edge and the center comes out clean.
This recipe was tested using a disposable “9×13-inch pan” that is actually slightly smaller than a true 9×13-inch pan. If you use a real pan that is true to size, your baking time will be significantly reduced. Check for doneness after 40 minutes.