Defrost and puree 300g of the blueberries, beat the whipped cream.
Beat the egg yolks and 80g of sugar with a hand-mixer over a simmering (almost boiling) water bath (Bain Marie) until frothy and the mass has doubled in volume and thickens. Mix the pureed blueberries with the sour cream and zests of lime. Delicately mix with the egg yolks. Now carefully fold in the whipped cream but do not mix completely. It’s OK if white blotches are still visible. Spoon into a middle-sized mould or several smaller ones and allow to freeze for at least two hours in the freezer.
Heat the rest of the blueberries with 20g of sugar in a pot, quench with port wine and let it cool down. Serve with the frozen parfait.