Preheat the oven to 350°F. Line a 8x8-inch baking pan with parchment paper.
To make the batter; In the bowl of a stand mixer fitted with the paddle attachment, add the coconut oil, brown sugar, coconut milk, and vanilla extract. Mix on low until combined.
Add the all purpose flour, coconut flour, baking soda, sea salt, and cinnamon and mix to combine. Then add the blueberries and mix on low until combined.
Transfer to the prepared baking pan and spread evenly. Bake for 22 to 27 minutes or until golden brown around the edges and a toothpick inserted in the center comes out clean. Allow to cool completely in the baking pan.
To make the frosting: In the bowl of a stand mixer fitted with the paddle attachment, add the shortening, powdered sugar, coconut milk, and blueberry powder. Mix on low until combined, then speed mixer up to high and mix for 2 to 3 minutes or until light and fluffy. Transfer to a piping bag with Ateco tip #864.
Remove the Blondies from the pan and cut into 16 squares. Pipe a squiggle of frosting onto the top of each square. Top with a little spoonful of blueberry jam (optional).
Store in an airtight container in the fridge for up to 5 days.