Make the blueberry jam first: Place blueberries, sugar, water and Certo into a deep saucepan (the deeper the better as when it's on a rolling boil it will rise!)
Place over a medium heat and stir to coat all the berries in the sugar. Bring to a rolling boil and continue to cook for 8-10 minutes, stirring occasionally so the mixture doesn't stick to the bottom of the pan.
After 8-10 minutes, the liquid should have reduced, the blueberries will have burst and be mushy and the mixture should coat the back of a metal spoon.
Leave to cool for 15 minutes before making the ice cream base: place the double/heavy cream into a large bowl, or the bowl of your stand mixer and beat on medium-high speed until you have stiff peaks. You want to pull your whisk up out of the mixture and have it hold its peak without the tip bending too much.
Add in the condensed milk and vanilla extract and fold in by hand until everything is well combined.
Take a 2lb loaf tin and spoon a third of the ice cream mixture into the tin. Spoon a third of the blueberry jam on top and use a skewer to swirl the two together. You want streaks of purple, but not the entire ice cream to be purple.
Add another third of the ice cream base gently, then repeat with a third of the blueberry jam. Repeat this one more time until you've used all of the ice cream - you may have 1tbsp or so left of the jam, it depends how much you want to use!
Place in the freezer for at least 6 hours, overnight if possible.
Serve in bowls with syrup, or in cones with sprinkles and a flake - or just alone! This creamy, sweet ice cream with bursts of intense blueberry jam is perfect alone too!
Blueberry Swirl Ice Cream will keep in the freezer for one month.