Line the bottom and sides of a 9-inch square baking pan with parchment paper, allowing excess parchment paper to hang over sides of pan.
In a small saucepan over medium heat, stir together blueberries, sugar, lemon juice, and cornstarch. Cook, stirring continuously and mashing blueberries, until mixture becomes a thick sauce, similar to a purée. Using the back of a spoon, press sauce through a fine-mesh sieve into a heatproof bowl. Set aside.
In the top of a double boiler set over simmering water, stir together white chocolate chips, sweetened condensed milk, and butter. Stir continuously to blend and until chocolate melts. Remove from heat.
Pour a third of chocolate mixture into prepared baking pan. Pour a third of blueberry sauce over chocolate layer. Using a knife, swirl chocolate and blueberry layers together. Repeat layers with remaining chocolate mixture and remaining blueberry sauce two more times, swirling blueberry layer each time.
Refrigerate until set, at least 3 hours.
To serve, using excess parchment paper as handles, lift fudge from pan and onto a cutting surface. Cut into 36 (1½-inch) pieces. Store in a single layer in an airtight container in the refrigerator and enjoy within a week.
Recipe by TeaTime Magazine at https://teatimemagazine.com/read-honeymoons-can-hazardous-latest-installment-amish-matchmaker-mysteries/