I'm playing a sneaky Poison Dusk Lizardfolk who dabbles a lot in poisons. I want to use a blowgun to inflict poisons on my targets, but I don't want them to know they've been damaged. Basically, imagine someone walking down the road, and then suddenly being paralyzed. I know there's rules for "sniping," or making a ranged attack and then hiding, but that doesn't stop the opponent from knowing they've been damaged. It just stops them from finding who did it.
The only ability I know that has anything like this effect is the Mosquito’s Bite skill trick from Complete Scoundrel. It requires 12 ranks in Sleight of Hand, so you’re talking 9th level before it’s available, and even then it only delays the target’s awareness of the attack for 1 round. But it means you can hit a target without them realizing they have been hit, and if your poison kicks in and knocks them unconscious before a round passes, maybe it’s good enough.
Another thing to consider here, if you can somehow justify it as a poison dusk lizardfolk, is being at least a 1st-level cleric of Lolth—because then you can take Lolth’s Caress from Drow of the Underdark. That lets you spend a turn undead attempt to trigger your poison’s secondary damage the turn after you hit them with it. Technically, they’ll become aware of your initial attack just as your turn starts, but then it’s your turn and you get to trigger Lolth’s Caress.
Also, if you’re unfamiliar, The Arsenic & Old Lace Handbook is must-read for any character interested in poison. As you’ll read there, the bare minimum you really, really want are ranks in Craft (poisonmaking), and the Master of Poisons feat—both from Drow of the Underdark. Seriously, that’s a really good book for poisons, and most of it’s not even drow-specific.