Place blueberries in a food processor fitted with the metal “s” blade or in a blender (if using a blender, 2 cups at a time for best results). Process until completely puréed and smooth. Press blueberry purée through a fine mesh sieve or chinois with a wooden spoon or spatula. Discard seeds — you will have approximately 3 cups blueberry purée.

Combine the blueberry purée with 3 tablespoons of the crème de cassis in a 2-quart non-reactive saucepan and place over medium-low heat. Cook until the mixture is reduced by half. Remove from pan and chill.

Place the sugar and 1 1/2 cups of the half-and-half in a 2-1/2 quart saucepan with a heavy bottom. Bring to a simmer over medium heat and stir to dissolve the sugar. Keep warm over low heat. Place the egg yolks in a medium bowl and whisk until thickened, about 2 minutes (may use a hand or stand mixer fitted with the whip on low speed). While whisking, slowly add 1/2 cup of the hot half-and-half/sugar mixture and whisk until blended.

Stir the egg mixture back into the saucepan; increase heat to medium. Stir the mixture constantly with a wooden spoon, until the mixture is thickened like a custard sauce and registers 180 °F when checked with an instant-read thermometer. Stir in remaining half-and-half, powdered milk, heavy cream and whole milk. Strain the mixture through a fine mesh strainer; stir in reserved chilled blueberry purée, remaining tablespoon of crème de cassis and vanilla. Cover and refrigerate at least 6 hours before continuing.

Turn the machine on; pour mixture into freezer bowl, and let mix until thickened, about 20 to 25 minutes. The gelato will have a soft, creamy texture. If a firmer consistency is desired, transfer the gelato to an airtight container and place in freezer for about 2 hours. Remove from freezer about 15 minutes before serving.