Black Lotus

Black Lotus is a rare Herb that can be gathered by Herbalists.

Source
Black Lotus can be found in Winterspring, Eastern Plaguelands, Burning Steppes, Silithus, and outside the instance portals in Dire Maul. It is rare in all of these places and never appears anywhere else. Requires 300 Herbalism to gather.

It can also very rarely be obtained from harvesting the corpses of plant mobs like Bog Lords in Outland. The chance to get a black lotus appears to increase with the monster's level, so chances are higher in places like Skettis, The Steamvault, Serpentshrine Cavern and heroic Coilfang Reservoir instances.

As an ingredient
This item is used in the following recipes:

Alchemy

 * Flask of Chromatic Resistance
 * Flask of Distilled Wisdom
 * Flask of Supreme Power
 * Flask of the Titans
 * Gurubashi Mojo Madness

Tailoring

 * Satchel of Cenarius

Black Lotus Spawns
Each of the spawn zones has a small set of spawn points for Black Lotus. Only one Black Lotus will spawn at a time in the zone. Check sites which focus on gathering WoW data to see maps of the spawn locations. It's estimated that the respawn rate is 1 hour, possibly spawning on the hour.

Spawn Maps

 * Burning Steppes
 * Eastern Plaguelands
 * Silithus
 * Winterspring

Reference/inspiration
The Black Lotus is probably either a reference to or inspired by Conan the Barbarian stories in which a highly venomous plant known as the Black Lotus first appeared. It could also be a reference to the card Black Lotus, from the Magic: the Gathering card game, in which the Black Lotus card is among the most useful and expensive cards in the game.