Line a 9″x5” metal loaf pan with plastic wrap, leaving 6” of overhang on each side. Make enough room in the freezer for the loaf pan to sit flat.
Make blackberry sauce: bring 1 cup of the berries, ½ cup of the sugar, and the water to a boil. Reduce heat to medium-low and simmer for 15 minutes. Strain through a fine-mesh sieve, then pour ¼ cup of the sauce in the bottom of the lined loaf pan, tilting the pan to evenly coat the bottom.
In a large mixing bowl, whip the cream until soft peaks form. Consistency should be silky whipped cream, not stiff enough for piping. Cover the bowl and place the whipped cream in the fridge.
In a medium, heat-proof mixing bowl, whisk the egg yolks and the other cup of sugar until the sugar has dissolved. Set the bowl over a medium saucepan with an inch of water (make sure the bottom of the bowl isn’t sitting in the water). Bring the water to a simmer, whisking steadily until an instant-read thermometer reads 170°F and the yolk mixture is beginning to thicken.
Remove the bowl from the heat and whip the mixture until light and fluffy. Fold the egg mixture into the whipped cream until fully combined, taking care not to overmix — it should stay nice and airy.
Pour the blackberry sauce into the whipped cream mixture in a swirl pattern but don’t stir it — you’ll get a prettier ripple in the semifreddo if you leave it alone. Pour half the blackberry-whipped cream mix into the loaf pan, sprinkle on a cup of the blackberries, then pour in the rest of the mix.
Fold the plastic wrap over the top of the pan and stash in the freezer for 6-8 hours or overnight. To release the semifreddo from the pan, set the pan in a tray of warm water for 30 seconds or so, then unfold the plastic wrap and flip the semifreddo onto a serving tray for slicing. Sprinkle on the remaining cup of blackberries and enjoy!