Line an 8-inch square pan with parchment, leaving overhang for easy lifting.
Crush freeze-dried blueberries into a fine powder using a blender or bag and rolling pin.
Warm condensed milk and butter in a saucepan over low heat, stirring until the butter melts.
Remove from heat. Add white chocolate chips and stir until completely smooth and glossy.
Stir in vanilla, salt, lemon zest, and the blueberry powder until evenly tinted.
Spread the mixture into the prepared pan and smooth the top.
Warm jam briefly if thick, then dollop over the surface and swirl with a knife.
Tap the pan to release air bubbles. Chill until firm, 2 to 3 hours.
Lift out using parchment. Cut into 25 squares, wiping the knife between cuts.