Add the blueberries, sugar and lemon juice to a small pot and place over low heat. Cook, stirring occasionally until the sugar is mostly dissolved.
Mix together the cornstarch and water together and stir it into the blueberries. Bring it to a simmer for two minutes until the mixture is thickened. Pour into a bowl chill in the fridge.
Meanwhile, add the heavy cream to a bowl and beat with a hand mixer until stiff peaks form. Continue to beat on low while streaming in the condensed milk. Stir in the lemon zest and juice.
Line a 9x5x3 loaf pan with plastic wrap so it hangs over all sides.
Assembly:
Remove 1/2 cup of the blueberry mixture to another container and reserve in the fridge until ready to serve.
Cover the bottom of the pan with 1/4 cup of the lemon ice-cream mixture.
Place a layer of graham crackers on the bottom so they fit snuggly in the pan.
Add 1/3 of the remaining lemon cream mixture and smooth it out to the edges.
Top with 1/2 of the remaining blueberry mixture.
Top with another layer of graham crackers, followed by 1/3 more of the lemon cream, followed by the rest of the blueberries and a final layer of graham crackers.
Top with remaining lemon cream mixture. Tap the loaf pan gently on the counter to remove any air pockets.
Fold the plastic wrap over the top and freeze for a minimum of four hours up to 24 hours.
Remove the cake from the freezer an hour before serving. Remove from the loaf pan and serve with the reserved blueberries over the top. Slice to serve.