Black Lotus requires a herbalist to be level 300 to gather and can be found at Winterspring, Eastern Plaguelands, Burning Steppes, Silithus, and Blasted Lands. It can be a loot from killing plant mosters like Bog Lord. Its seems that only one Black Lotus can spawn in a zone at a time. and will respawn once every hour. It is indeed a very rare herb to find.
See maps below for a chance to farm Black Lotus in Wow.