This dessert swirls ripe fruit in a sweet-tart base of vanilla-creme fraiche custard.
Ideally, chill the custard overnight. If you're using an ice-cream machine with a removable double-walled canister, store the canister in the freezer for at least 24 hour hours between uses.
Combine half and half, ⅔ cup sugar, and vanilla bean in a 3- to 4-quart pot over medium heat, until sugar dissolves and mixture is just beginning to steam, about 5 minutes.
Meanwhile, in a small bowl, whisk egg yolks briefly to combine. Stir ⅓ cup hot half-and-half mixture into egg yolks to warm them; then add yolk mixture back to rest of half-and-half mixture in a thin stream, stirring continuously. Keep stirring over medium heat until custard begins to bubble and thicken and reaches 175º on an instant-read thermometer. Strain custard through a fine sieve, reserving vanilla bean; return it to the mixture once strained. Cover mixture with plastic wrap and chill in your refrigerator until fully cold, at least 3 hours, preferably overnight.
About 30 minutes before you're ready to freeze your ice cream, mash blueberries roughly; mix in 2 teaspoons sugar and stir to dissolve. Freeze blueberry mixture just long enough for juice to solidify, 20-30 minutes. Remove vanilla pod from custard and stir in creme fraiche. Freeze in an ice-cream maker according to the manufacturer's instructions. Spoon frozen custard into a chilled bowl and gently fold in blueberries to produce purple streaks through the creamy base. Don't overmix. If you like, pack ice cream into a plastic or glass container and store in your freezer for two hours to harden. Let ice cream sit out 5-10 minutes before scooping.