No, you only need to be in exp range when the mob dies to learn it. This is true for all blu spells. This also happens to be true for TH, you must be within exp range when the mob dies.
But Maruraba is correct, you must cast this into the mobs face, you can be facing any which way you like, but the mob must be facing in your direction. Eg. Crawler casts cocoon, you run behind tank, and cast Blank gaze, and pop, crawler loses def boost. One less job for your over worked redmage.
Just like the mob spell, you must be facing your opponent for this spell to remove a beneficial effect. Try it from the side or behind, and it will not work.