Where to go Plaguebloom Farming in Classic
Plaguebloom only grows in three zones: Eastern Plaguelands, Western Plaguelands and Felwood, with the latter also being great for Gromsblood farming. Plaguebloom requires a skill level of 285 in Herbalism in order to pick it. In addition to Plaguebloom farming, these zones also have a bunch of Dreamfoil, while the Eastern Plaguelands even has Black Lotus.
The Route in the Western Plaguelands is a lot shorter, though it can be better if you have either Skinning or Mining as your second profession, as the route works great for Thorium and Rugged Leather farming as well. The Eastern Plaguelands can be better because of the Black Lotus, but there might be more competition there, too. So, here are the routes: