Preheat oven (electric cooker: 180 °C/circulating air: 160 °C/gas: see manufacturer). Line the bottom of a springform pan (20 cm Ø) with baking paper. Separate the eggs. Put the egg white in a cool place. Break chocolate into pieces and melt in a hot water bath. Mix cream cheese and egg yolk. Add liquid chocolate while stirring.
Beat the egg white until stiff and fold into the cheese mixture in 3 portions. Fold in frozen blackberries. Pour into the mould and smooth down. Bake in a hot oven for about 30 minutes. After approx. 15 minutes reduce the oven temperature (electric oven: 150 °C/circulating air: 130 °C/gas: see manufacturer).
After the end of the baking time, let the cake rest in the switched off oven for about 15 minutes, then remove it and let it cool down completely. Dust with icing sugar.