In a medium sauce pan, combine blueberries and ½ cup sugar, and mash them together. Cook stirring occasionally until jam consistency. Remove from heat and blend with an immersion blender. Add lemon juice and gelatin, and return to medium heat, whisking until gelatin dissolves and mixture is hot (not boiling). Remove from heat and pour into another bowl to cool to room temperature. Watch carefully because once it cools, or it will thicken and become difficult to blend.

Whip heavy cream with powdered sugar on high speed until it forms peaks. Gently fold in blueberry sauce. Cover, or transfer to piping bag, and refrigerate 3 hours or overnight