Prepare the necessary ingredients. Wash and dry the blackberries. Wash the eggs and separate the yolks from the whites.
Beat the yolk. Place the yolk in a saucepan and add sugar. Beat with a mixer until the mixture turns white and fluffy. Place the mixture on low heat. While continuing to whisk, pour in the milk in a thin stream. The mass should thicken and increase in size. Remove from heat and cool.
Prepare the berry-cream base. Place the blackberries in a blender bowl and blend until pureed. Whip the cream in a separate bowl until stiff peaks form. Gently combine blackberry puree, whipped cream and cooled egg mixture.
Refrigerate the blackberry ice cream. Pour the ice cream into a container or pour into molds and place in the freezer for 3 hours. Every hour, remove and stir the sundae, breaking up the frozen edges and stirring them into the mixture to prevent ice crystals from forming in the ice cream.
Before serving, garnish the sundae with fresh blackberries.