Lightly oil an 8 1/2-by-4 1/2-by-2-inch loaf pan and line it as neatly as possible with plastic wrap, allowing enough to drape over the sides to later cover the top. (I lay two pieces of plastic wrap across the pan, overlapping in the middle.) Place the pan in the freezer.
Put the egg yolks (reserve 1/3 cup of the egg whites and discard or save the rest for another use), 1/4 cup of the sugar and the vanilla seeds in a medium heatproof bowl and beat with a handheld mixer fitted with the beater attachment for 2 minutes, until light yellow and thickened. Place the bowl over a pan of simmering water and beat the mixture continually for 3 minutes, until doubled in volume and warm to the touch. Off the heat, beat the mixture for 1 minute.
In the bowl of an electric mixer fitted with the whisk attachment, add the reserved egg whites, the remaining 1/4 cup of sugar and the cream of tartar and beat on high speed until the whites form firm but not dry peaks. Fold the egg whites into the egg mixture with a rubber spatula.
Without washing the mixer or whisk, pour the heavy cream, vanilla liqueur (if using) and vanilla into the bowl and beat on high speed until it forms soft peaks. Fold the whipped cream carefully but thoroughly into the egg mixture. Pour the mixture into the prepared pan, cover with plastic wrap, and freeze for at least 4 hours.
Place the raspberries, sugar and 1/4 cup water in a small saucepan. Bring to a boil, lower the heat and simmer for 4 minutes.
Pour the mixture and the preserves into the bowl of a food processor fitted with the steel blade and process until smooth. Stir in the framboise and chill.
To unmold, dip the pan in a bowl of hot tap water for 8 to 10 seconds and turn the semifreddo upside down onto a rectangular serving plate. Peel off the plastic wrap. (You can cover and refreeze the semifreddo for a few hours on the serving plate.) Pour several tablespoons of raspberry sauce on each dessert plate, cut 1-inch-thick slices of the semifreddo and place them in the center of the sauce. Sprinkle each serving with blueberries and serve immediately.