Line a 9x5" loaf pan with a plastic wrap, leaving overhang on all sides.
With an electric mixer on medium-high speed, beat mascarpone cheese, powdered sugar, lemon zest, and lemon juice together until light and fluffy, about 4 minutes.
On low speed, slowly add chilled heavy cream to mixture. Once added, increase to medium speed and beat until medium peaks form. Set aside 1/3 of the mixture into a small bowl, cover with plastic and chill. Use remaining 2/3 for the next steps of assembly.
Place a single layer of graham crackers on the bottom of the lined loaf pan, break as needed to fit. Evenly spread a quarter of the cream over the graham crackers. Top cream with ½ cup of blueberry sauce and evenly spread. Evenly spread another 1/3 of remaining cream over blueberry sauce and top with a single layer of graham crackers. Repeat layers again and evenly spread ½ of remaining cream over graham cracker and top with ½ cup of blueberry sauce. Finish with the remaining cream and one last layer of graham cracker.
Fold excess plastic wrap tightly over the cake.
Freeze overnight, or at least 6 hours. Cover and refrigerate remaining ½ cup of blueberry sauce.
Uncover cake and invert onto a platter. Peel off and discard plastic.
Spread the tops and sides of cake with reserved bowl of mascarpone cream mixture. If it looks loose, whip mixture to stiffen. Chill in freezer for 15 minutes.
Mix blueberries with reserved ½ cup of blueberry sauce and spoon to the top of the cake. Peel zest from lemon and slice into fine strips. Sprinkle over blueberry topping.