Some research on arrow poisons took me here for some interesting notes and wise counsel, so I'll exhume the necro thread to bring it up to a full 16 years.
The three most important things to remember are safety, Safety, SAFETY!
Safety #1 - Always use a blowgun that is longer than the distance from your mouth to your own foot, and remember: to fire, exhale, DON'T INHALE!
Safety #2 - If you plan to eat what you kill, seek out poisons that will be denatured (i.e., deactivated) by cooking, or at least diluted enough (e.g., in a stew) that they will never reach the diner at a concentration high enough to be dangerous. Some deadly blood poisons can be eaten safety, even without cooking or dilution, but others are systemic poisons still dangerous upon digestion, even AFTER cooking and/or dilution.
Safety #3 - Consider the environmental hazard of a lost dart. Children or cute, fuzzy kittens may frolic barefoot tomorrow where you hunted varmints yesterday! Some poisons will degrade pretty quickly when exposed to oxygen, water, sunlight, etc., while others are quite persistent. The persistent kinds are generally the least safe for eating, so choose something that will degrade biologically, thermally, or in direct sunlight if at all possible.
With a degradable poison, you probably can't keep it active in long storage, but you really don't want to store it on pointy things like darts anyway. Ideally, you would procure / prepare the poison right before the hunt, dip the dart tip just before (very carefully) inserting it into your blowgun, then a-hunting ye shall go. If you finish your day of hunting with a tipped dart still loaded, dispose of it carefully, like maybe via a gentle shot into a pot of water that you will then boil, then rinse the blowgun out with the boiling water. Most people can and will use plenty of caution when they first load the blowgun, but it's much easier to come back to that blowgun a few weeks or months later and forget what it was loaded with last time. Or maybe your semi-retarded little brother / child / neighbor is the next guy to give the blowgun a try, and maybe he thinks it would be amusing to try it out on your rear end, the family dog, a cute, fuzzy kitten, etc., etc.
Also, avoid poisons known to transfer through unbroken skin. If you scratch yourself with a dart properly loaded to bring down a squirrel, chances are you'll live. But if you spill the whole bottle on your hand, and it's of a type that readily transfers through human skin, you may not. Same caution for inhalation hazards.
Above all, DO YOUR RESEARCH. Even with a dozen college chemistry courses behind me (and all A's except for f***ing O-Chem Lab 2), I approach the preparation and use of such materials with great care and more than a little fear. To be useful on the tip of a dart, a poison must be so CONCENTRATED that it becomes far more dangerous than the exact same compound might be in, say, a diluted rodent killer bait, a lawn & garden pesticide, or (as in the case with nicotine) in a cigarette. This is why I will make no specific recommendations. Once you've identified some candidates, there are plenty of useful references on the Internet and at the local Library to determine whether a given candidate will suit your needs, and, if so, what is needed to handle it safely. But I'll also join those who tactfully suggest that the game you seek can be brought down by purely ballistic projectiles; no need to resort to chemical warfare!
Any knife never nicked you? I think poison was not widely used to hunt because it can get the hunter just as easily.