Prepare the lemon curd. In a medium saucepan, stir together sugar and cornstarch. Stir in the lemon juice, lemon zest, water, and salt. Cook over medium heat, stirring constantly, until thickened. Remove from heat and gradually stir in half of the egg yolks, mix well until combined. Stir in the rest of the egg yolks. Return to the heat and cook for 2 minutes, stirring constantly. Remove from heat. Stir in the butter, mix until incorporated. Cover with plastic wrap, make sure it touches the surface of the lemon curd; this will prevent the curd forming a skin. Refrigerate until completely cool.
Mix the blueberries with the blueberry jam. Set aside.
Prepare the whipped cream. In a large bowl, beat the cream, sugar, and vanilla. Whip until soft peaks form. Do not over beat.
Assemble the trifle. Place one layer of cubed pound cake at the bottom of a trifle dish. Top with a layer of blueberries. Add dollops of lemon curd and whipped cream. Repeat the process of layering ending with whipped cream.
Decorate the trifle with lemon slices, fresh blueberries, and mint.