In a medium, heavy saucepan combine egg yolks, milk, sugar, and nutmeg. Cook and stir over medium heat until mixture coats a metal spoon. Remove from heat. Place pan in bowl of ice water; stir 2 minutes.
Stir in whipping cream and vanilla. Cover and chill 4 to 24 hours. Place blackberries in individual bottles or pitcher. Pour eggnog over berries. Makes 8 servings.