But... you're not striking with the blowgun... >.> you're striking with the dart...
As written, you are attacking with the blowgun. Sure, you use ammunition to attack with the blowgun, but that does not change the fact that you are attacking with the blowgun. A +1 blowgun gets +1 to attack and damage with the darts. In the same way, one could argue that , as written, the weapon that gets the poison is the blowgun and so the darts all get the poison form the blow gun.
You're welcome to applying the poison to the blowgun, but don't be surprised if you end up being poisoned...As written, you can apply the poison to the blowgun itself instead of the darts, and it will then last for the encounter.
Some of the poisons work only for the first attack and some work for the entire encounter. This doesn't make much sense. It should be one or the other.
I agree that ranged attacks are seriously hosed in this. Especially when you give the Assassin at-wills that use specific weapons (such as the blowgun).