Imperfections mar the beauty of creation. In a battle among the gods themselves untold aeons ago, a discharge of divine might changed the weather patterns of the world so that once every few decades, an oily black rain pours out of particularly dark clouds. Whether by design or by mere mischance, this rain signals a period during which inviolate power severs the link between the mortal and the divine worlds. Even the gods themselves can do nothing to remedy this. Clerics lose all spellcasting ability, wards are suppressed, and so on.
During the time of a black rain, all characters lose the ability to cast divine spells or use divine spell completion items. Clerics lose the ability to channel positive or negative energy, and divine-based classes such as the paladin or druid lose supernatural or spell-like abilities (in the paladin's case, for example, this includes detect evil, divine grace, lay on hands, divine health, aura of courage, smite evil, remove disease, and turn undead).
Further, divine spell effects currently active when the rain starts, including permanent divine spell effects such as glyphs of warding, hallow, or bull's strength cast on a character, are all suppressed for the duration of the rain.