If you're searching for a show stopping dessert (that is secretly easy to make) to serve company, look no further. Each gorgeous — and delicious — tricolored slice is a canvas of decorative swirls: vibrant orange, deep purple, and creamy white.
Put sorbet and ice cream in refrigerator until evenly softened, 45 minutes to 1 hour.
Meanwhile, purée blackberries, sugar, and cassis in a blender until smooth, then strain thourough a fine-mesh sieve set over a bowl, pressing on and then discarding solids. Freeze to thicken slightly until ice cream is ready, 20 to 40 minutes, then stir until smooth.
Lightly oil loaf pan, then cut a piece of parchment to fit bottom and long sides of pan, leaving at least 3 inches of overhang on each side.
Fill pan decoratively with spoonfuls of sorbet and ice cream, pressing down and filling empty spaces with blackberry purée as you go. Smooth top, pressing down with back of spoon to eliminutesate air spaces, then fold parchment flaps over top and freeze until solid, at least 3 hours.
To unmold, run a thin knife along short sides of pan to loosen mosaic, then open parchment and invert onto a flat serving dish, discarding parchment.
Cut mosaic into 1/2-inch-thick slices.
Cooks' note: Mosaic can be made 5 days ahead and frozen, covered with plastic wrap.