Put white chocolate chips, sweetened condensed milk, and butter in a large microwave safe bowl.
Microwave for 2 minutes, stir everything together as much as you can.
Microwave in additional 20 second increments (up to 1 minute) until you can stir the mixture totally smooth.
Scoop about 1/3 of the fudge mixture into a 9x9 pan, lined with parchment paper.
Spoon about 1/3 of the berry sauce over the top.
Swirl the berry sauce and the fudge together with a knife.
Repeat with another third of the fudge, then the blueberry sauce, then swirl again.
Spread the rest of the white chocolate fudge over the top, and spoon the remaining blueberry sauce on top of it. Swirl it again to make a pretty swirl of berry sauce and fudge.
Place fudge in the fridge to harden for at least 2 hours, or until totally set.
Slice fudge into small pieces and serve.
Store any extra fudge in an airtight container for up to 1 week.