Preheat your oven to 350 F. Line a square brownie pan with parchment paper.
In a large bowl, mix the first 5 ingredients. Then add the egg, syrup, oil, milk and zest and mix until just combined and smooth. Lastly, fold in the blueberries. Transfer the batter to your pan and spread out with a rubber spatula.
Mix together the crumble ingredients in a small bowl. Top your batter with it and gently spread out.
Bake for 40 minutes or until toothpick comes out mostly clean. Allow to cool 20 minutes to remove and slice.
Notes
In the cake ingredients, you can substitute in 2 cups oat flour instead of the almond and coconut flours.