Blue herb is a SP restorative item which cannot be bought from NPCs. (It is not sold in official servers but I'm not sure about private servers though.) It is commonly used by alchemist classes for brewing blue potions and other SP restoring items. Through random exploring of different maps, I happen to find a few good spots where you can get blue herbs. (This is from PRO Valhalla server)
gef_fild10 48,349 (Orc map)
You can find three blue plants at the back of the entrance of Orc Dungeon. I usually leave one of my characters here and come back after 8 to 10 minutes. It is better if you use a thief class because of Steal to increase the number of items that you could get from the plants.
gef_fild02 218,209 (Coco map)
This place is 1 map south and 2 maps west of Prontera. Around 3 to 4 blue plants spawn in this area. Unfortunately, this must have been a popular spot now for farming blue herbs since bots are wandering around that area lately like that thief there. -_-
juno_fild01 66,369 (Geographer map)
I really wouldn't suggest this place as a farming spot since it's quite far and only 2 blue plants spawn in this area and there are also strong monsters in this map. I happen to see these plants while doing Kiel Hyre quest. This place is 1 map north of Al de Baran.
Now if you prefer to hunt down blue herbs from monsters, try killing these:
(I suggest you use bubblegum for higher chances of getting blue herbs)
Mavka
(mosc_dun03)
Mavka also drops crystal mirror, witherless rose and singing flower which you can sell to NPCs or players for a pretty good amount of zeny.
Goat
(ein_fild06, juno_fild06, juno_fild07)
Goat drops almost all kinds of herbs - blue, yellow, red, green herbs
