To make lemon curd: In medium saucepan, stir sugar and cornstarch. Stir in lemon juice, lemon zest, water and salt. Cook over medium heat, stirring constantly, until thickened. Remove from heat and gradually stir in three egg yolks; mix well until combined. Stir in remaining egg yolks. Return to heat and cook 2 minutes, stirring constantly. Remove from heat.
Stir in butter; mix until incorporated. Cover with plastic wrap, touching surface of lemon curd to prevent curd forming skin. Refrigerate until completely cool.
To make whipped cream: In large bowl, beat cream, sugar and vanilla until soft peaks form. Do not overbeat.
To make trifle: Mix blueberry jam with 12 ounces fresh blueberries. Place one layer cubed pound cake in bottom of trifle dish. Top with layer of blueberries. Add dollops of lemon curd and whipped cream. Repeat layering ending with whipped cream.
Decorate trifle with lemon slices, fresh blueberries and mint.