Vials of Blessed Milk were frequently found near a herd of blessed cows, which were located in the Frigid River Valley. The milk itself, which was used as the drink to go along with Nanny Beeble's demon warding cookies, was always surrounded by a strange, glowing aura.

The blessed milk  has been reported as early as 723 GUE and as late as the Great Monster Uprising of the Second Age of Magic, where a Vial was obtained by the Grand Wizard and his underlings of G.U.E. Tech.

SOURCE(S): Legends of Zork