For the filling: in a saucepan over medium heat, combine the blueberries, salt, and sugar. Cover, and let the berries start to soften and release their juices, stirring occasionally. Once berries have broken down and the mixture is becoming more like a sauce, reduce heat and let simmer for 7-8 minutes, covered. Meanwhile, in a small bowl, combine the lemon juice with the arrowroot and agar, and stir through until combined (it will be thick). Get ready to add the arrowroot/agar blend to the blueberries by first removing 2-3 tablespoons of the warmed blueberries juices and stirring into the arrowroot slurry, one tbsp at a time. This will help loosen that mix and also temper the arrowroot and agar, making it less likely to clump in the sauce. After it has loosened, add it to the blueberry sauce, immediately stirring it through. Increase heat slightly to bring mixture back to a boil for just a minute or so. Then, remove from heat and transfer mixture to a bowl and let chill in refrigerator for about 20-30 minutes, stirring occasionally to prevent a ‘skin’ from forming on top of the mixture.
Meanwhile, prepare the crust: Preheat oven to 350°F (176°C). In a large bowl, combine all the dry ingredients including zest. In a small bowl, combine the maple syrup, vanilla, and coconut butter. Add the wet mixture to the dry, and stir through until well combined; the consistency will be crumbly yet hold together somewhat when pressed. Transfer about 2⁄3 of the mixture to a lightly oiled (or lined with parchment paper) 8x8 pan and press in evenly. Pour the chilled (can still be slightly warm) blueberry mixture on top, then sprinkle on the remaining crust mixture. Bake for 25 minutes, until the blueberry filling is bubbly and the crust is golden brown in spots. Transfer to a rack to cool, and then to the fridge to cool completely before cutting into squares.