This Netherbloom farming guide will show you the routes that I use for farming Netherbloom. There were no major changes to herbs at Outland, so this guide can be used for Burning Crusade Classic (TBC) and Retail WoW.
Netherbloom is generally found out in the open. Gathering a Netherbloom will have a chance to grant a +50 a random stat for 15 minutes and you will occasionally get between one and three Motes of Mana in addition to the herb.
Netherstorm is the only place where you can farm Netherbloom, it doesn't grow at any other zones.
The respawn rate is fast enough that you don't have to loop around the whole zone. I mainly used this farming route because it's very easy to follow and you get a lot of Netherbloom.
This route loops around the whole zones, checking every possible herb spawns. You should use this if there is a lot of other people farming in the zone.