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STEP 1: Line a 9-by-5-inch loaf pan with 2 sheets of plastic wrap, leaving an overhang on all sides to make unmolding easy.
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STEP 2: Pulse the blackberries and raspberries in a food processor just until broken up.
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Pour the berries through a coarse strainer set over a bowl, pressing hard on the solids to extract as much liquid as possible.
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STEP 3: Bring the sugar, water, and salt to a boil in a small saucepan over high heat, stirring until the sugar is dissolved.
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Boil for 2 minutes.
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Remove from the heat.
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STEP 4: Beat the egg whites with an electric mixer on medium speed in a large bowl until foamy.
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Increase the speed to medium-high and, with the mixer running, slowly pour the hot sugar syrup into the egg whites, being careful to avoid the beaters.
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Continue beating for 8 to 10 minutes, until the egg whites are cooled to room temperature.
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STEP 5: Beat the cream on medium-high speed in a large deep bowl until it forms stiff peaks when the beaters are lifted.
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With a whisk or a rubber spatula, fold the berry puree into the egg white mixture until almost blended, then fold in the whipped cream.
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STEP 6: Spoon into the loaf pan and spread into the corners with a rubber spatula.
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Cover with the plastic and freeze for at least 6 hours, or overnight.
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STEP 7: About 15 minutes before serving, remove the semifreddo from the freezer.
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Fold back the plastic wrap, invert the semifreddo onto a serving platter, and peel off the plastic.
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Let stand for 10 minutes to soften slightly.
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Serve, cut into thick slices, with the berries, if using.
