Using a blender or food processor, process the graham crackers until they are very fine crumbs.
Pour the crumbs into a bowl and stir in the butter and sugar until everything is well combined.
In a large 9" springform pan or an OXO pie dish press the crumb mixture into the bottom and up the sides. Flatten with a glass. Place in the freezer for about 10-20 minutes to chill.
Using an electric mixer on medium high, beat the cream cheese until smooth. Pour in the sweetened condensed milk and beat until well combined. Beat in the lemon juice, lemon zest and lemon extract, again beating until completely incorporated.
Pour the filling in the prepared crust. Smooth the top and cover with plastic wrap. Chill in the refrigerator about 3 hours or until firm.
Make Blueberry sauce for top. In small bowl, mix the cornstarch and 2 tablespoons water together. Set aside.
Place 1 1/2 cups blueberries into a small saucepan, add 4 tablespoons water, sugar and lemon juice. Heat over medium heat until the mixture boils and then blueberries begin to burst, stir frequently. Add the bowl of dissolved cornstarch and allow the mixture to come to a boil again. Allow to boil hard for about 30 seconds to 1 minute stirring constantly. Lower heat and then add the other 1 1/2 cups of blueberries. Simmer until your sauce has reached desired consistency. Allow to cool to room temp.
Spread over chilled cheesecake and serve! YUM