In a large-size bowl, add all the dry ingredients and mix until combined. Add all of the wet ingredients except the blueberries and mix until the batter is combined and smooth.
Pour the cake batter into the lined baking tin. Scatter blueberries over and press them into the cake batter.
Bake the cake for around 25 minutes or until a skewer can be inserted into the middle of the cake and it comes out clean. Allow the cake to rest in the cake tin.
To make the two-toned frosting:
Add the blueberries to a small saucepan over medium-high heat. Boil for 15 minutes and mash them with the back of a fork. Boil until the blueberries have softened and their juice has thickened.
Add 1/4 of the blueberries to a blender with the rest of the frosting ingredients. Blend on high for 3 minutes or until extremely smooth.
Pour out 1/2 of the frosting into a bowl. Add the remaining blueberries to the blender and blend until extremely smooth. Set aside both frostings in the fridge to chill.
When the sheet cake has completely cooled, dollop both frostings on top of the cake. Use the back of a spoon to marble the frostings and to spread it on the cake's surface. Avoid over-mixing the frosting to ensure the the colour of each frosting is still noticeable.
Serve the sheet cake immediately. Alternatively, cover the cake tin and store in the fridge for up to 5 days.