Heat a saucepan over low/medium heat.
Stir 2 cups of blueberries, sugar, lemon juice, 3 tablespoons water into pan and bring to a boil.
Let boil for 3-4 minutes, then add remaining 1 cup of blueberries. Stir.
In a small bowl, mix 4 tablespoons water and corn starch together. Add to blueberries and stir.
Continue cooking until sauce thickens. Remove from heat and let cool.
Once cooled, spread onto the top of the cake and, using the end of a wooden spoon, gently poke the filling into the holes that were previously made.
Cover with plastic wrap and place in fridge to chill (this makes frosting the cake over the blueberries easier if the cake has been chilled).