1. Combine sugar and cornstarch.
2. In a medium saucepan on medium-high heat, cook blueberries, sugar, cornstarch and lemon zest and juice for 20 minutes. Blend half of the mixture so you are left with two versions of blueberry sauce: chunky and smooth.
3. While blueberries cook, combine flour, oats, sugars, salt and margarine until crumble forms.
4. Press mixture into 9×13-inch tray in a quarter-inch layer. It will not cover the entire pan.
5. Preheat oven to 350 degrees Fahrenheit and bake for 20 minutes or until golden brown. Let cool and crumble mixture with your hands.
6. Scoop out half the ice cream and place in a bowl. Pour a third of the smooth blueberry sauce and a third of the chunky blueberry sauce and a half cup of crumble, and swirl into ice cream.
7. Top with remaining ice cream and repeat process with a third of each sauce and a third of the crumble, reserving remaining sauce and crumble for serving.
8. Scoop out two scoops of ice cream per person and drizzle with additional blueberry sauce, chunky blueberries and remaining crumble.