Production Rune used for Alchemy Recipes of level 11 and higher.
Every mob in the game has a chance to drop Production Runes.
Note: If your lvl is too high dropping Blend Rune from low level monsters is going to be difficult. At lvl 50, to drop 1 Blend Rune from a Wild Spider will take min. 30 mins. My suggestion is to create a new toon.