The 1000 Needles spell is nice to have as a Blue Mage, and like any other spell, you need to find an enemy that performs it, then defeat that enemy to learn the spell. You do not have a 100 % chance to learn the spell upon defeating the enemy, and while the exact % chance to learn the spell is unknown, the learn chance seem to be pretty low.
There are two enemies in the Open World that uses the 1000 Needles spell, and these are Sabotender Bailaor in Southern Thanalan (x16,y15) (level 26), and Sabotender Bailaor in Flower Power Levequest in Southern Thanalan (x18,y13) (level 24).
So you should be at least level 24 before you attempt to learn this spell on your Blue Mage.
In order to farm this spell, just keep battling these enemies until they use the spell, the defeat them. Either of the two mobs listed above works fine, but most people prefer to do the enemies that are the lowest level.
Learning it seem to be a grind for most people, since it's a high ranking spell, and thus has a low drop chance.