Make the Blueberry Syrup
Add blueberries and water to a saucepan over medium heat.
Mash the blueberries with a fork or potato masher and simmer for 2 minutes.
Strain the mixture through a fine mesh sieve into a bowl, pressing to extract as much juice as possible. Discard the pulp.
Return the juice to the saucepan and stir in sugar and lemon juice.
Bring to a boil, then reduce heat and simmer for 3 minutes. Let cool completely.
Prepare the Cream Filling
In a medium bowl, whip heavy cream with sugar until stiff peaks form, about 3–5 minutes.
In another bowl, whisk mascarpone cheese and vanilla extract until smooth.
Fold one-third of the whipped cream into the mascarpone mixture. Repeat with the remaining whipped cream until fully combined.
Fold 3/4 cup of the cooled blueberry syrup into the mascarpone mixture until smooth.
Assemble the Tiramisu
Stir a shot of espresso into the remaining blueberry syrup.
Dip each ladyfinger into the blueberry-coffee syrup for about 10 seconds and arrange in a single layer in your serving dish.
Spread half of the blueberry mascarpone mixture over the ladyfingers.
Add another layer of dipped ladyfingers, then spread the remaining cream mixture on top.
Cover tightly with plastic wrap and refrigerate for at least 6 hours or overnight.
Make the Blueberry Curd
In a bowl, whisk together water, cornstarch, and egg.
Add blueberry jam to a saucepan and strain the cornstarch mixture into the pan. Stir well.
Cook over medium heat for about 5 minutes, stirring often, until thickened.
Remove from heat and stir in lemon juice and butter until smooth.
Allow the blueberry curd to cool completely.
Finish and Serve
Spread the cooled blueberry curd evenly over the chilled tiramisu.
Garnish with fresh blueberries and mint leaves before serving.