Stir together graham cracker crumbs, melted butter, and sugar until the texture resembles wet sand. Press firmly into the bottom of a 9-inch pie dish or 8x8 inch pan, using the bottom of a measuring cup to create an even layer. Refrigerate while you make the filling.

In a large bowl, beat softened cream cheese on medium speed until completely smooth, about 2 minutes. Add sweetened condensed milk, lemon juice, and 1 tablespoon lemon zest. Beat until well combined and no lumps remain, scraping down the bowl once or twice.

In a separate large bowl, whip cold heavy cream on medium-high speed until stiff peaks form, about 3-4 minutes. The cream should hold its shape when you lift the beaters - do not overwhip or it will become grainy.

Gently fold about one-third of the whipped cream into the lemon mixture to lighten it, then fold in the remaining cream in two additions. Use a rubber spatula and work slowly, cutting through the center and turning the bowl, until no streaks of white remain. The mixture should look like thick, pale yellow clouds.

Spread the lemon fluff evenly over the chilled crust. Cover loosely and refrigerate for at least 2 hours, or until set and sliceable. The filling will firm up as it chills but stay soft and mousse-like.

Just before serving, scatter fresh blueberries over the top and sprinkle with remaining lemon zest. Slice with a sharp knife, wiping clean between cuts for neat slices.