Combine the blueberries, sugar, cornstarch, and lemon zest in a saucepan over medium heat.
Stir the mixture until the sauce thickens and the cornstarch becomes translucent (around 10 minutes.)
Remove from heat and allow to cool to room temperature.
Once cooled, I blitzed my mixture in the food processor to remove any big blueberry chunks.
Cream the butter and icing sugar together until well mixed.
Add the cream cheese in cubes slowly until incorporated. Add in vanilla extract and continue mixing for 5 minutes until the icing is light and fluffy.
Gradually add the blueberry sauce and gently fold into the cream cheese frosting.