Which would of been a really cool weapon mastery to give it.
But it as Vex and Loading.
Thinking about this point, I'm not sure what would be a good Mastery for the Blowgun.
Cleave - Doesn't make sense since the needles tend to stick in the target. Unless you somehow managed to blow the needle hard enough to go through the target into the person behind them.
Graze - I suppose this makes sense. But at 1+DEX damage, Graze would only be doing 1 less point of damage (just the DEX straight up).
Nick - Nope. Not a Light weapon.
Push - Like Cleave, I doubt you could blow with enough force to knock the target backward.
Sap - Seems a bit of a stretch that the needle will be enough of a distraction to cause Disadvantage for the target.
Slow and
Topple - Don't make sense in and of themselves. Maybe if some poison were on the needle that would slow or topple the target, they would make sense. But, then, that would be the effect of the poison itself.
Vex - Unlike Sap, being hit with a needle from out of nowhere might be enough of a distraction to allow you to make a second attack (once you're level 5 or higher to utilize Extra Attack). But it seems one of the two reasons to use a blowgun is to deliver a poison rather than doing damage, so a second attack (with the blowgun or another weapon) is pointless. The only other reason I can think of to use a blowgun would be stealth, in which case you'd probably want to slip away from the scene instead of sticking around to make another attack.