Preheat the oven to 350°F. Line a 9x9-inch baking pan with parchment paper and lightly spray the pan.
Using a microplane grater, zest the lemon. Then, cut it in half and squeeze about 2 tbsp of lemon juice and set aside.
Peel and chop the pineapple.
Mix the flour, baking powder, baking soda, salt and poppyseeds in a bowl.
Add the lemon zest and mix until incorporated.
Add the eggs, oil and lemon juice, then stir until the ingredients are combined, but do not overmix.
Gently fold in the frozen blueberries and chopped fresh pineapple.
Spoon the batter into the prepared baking pan and spread it evenly.
Bake for 30–33 minutes, at 350 degrees Fahrenheit, or until the cake is set and lightly golden and a toothpick inserted into the center comes out without wet batter.
Let cool completely.
Serve as is or with a lemon glaze. Make the glaze by mixing the 2 ingredients for the lemon glaze together and drizzle over the cake.