Blow darts have the advantage of being light weight and a low bulk ammo. Also the blowgun can be used from hiding and usable from within deep cover. Also if you know anything about the physics of bows, small creatures with low strength would find little benefit in using bows and arrows. Blowgun darts also have the benefit of doing little damage or causing little blood loss, so if capturing something that is going to be eaten much later, the hunter is risking spoilage or infection.
This is good idea if you want to capture the players for some role playing or story line opportunity. Do you want them to be captured so they have to use their guile (persuasion, intimidation, diplomacy) or use their James Bond like abilities and stealth once they've been taken into camp. Perhaps the goal is to put players in a particular location, introduce and rescue an important NPC (fellow humanoid captive...Or you want to introduce an animal companion to a particular player. "After rescuing the jaguar, it seems to continue to follow you at a distance as you retreat into the jungle".), or initiate some story hook where the chieftain imparts some important information needed by the party amidst his explanation of how they are going to be the main dish in their forthcoming feast.
A home brew for the Grung blow darts is that they don't do any damage. So I don't spend time rolling for damage, just to hit. And characters in full plate armor are never affected except on a natural 20 hit (hit exposed area of flesh).
By 5e rules, the poison the grung excrete does not need to reach the blood stream as it is described as grapple or contact is all that is needed for the player to make a save. And it can be applied to weapons. no mentionn of breaking skin or ingestion is stated. But perhaps the poison they excrete on contact has a different status effect than when it's introduced to the cardio vascular system.
Furthermore, if you as the DM decide to make it more entertaining, the grung's poison doesn't have to do damage either, but may simply be a status effect. Examples: sleep, paralyzation, confusion, hallucination inducing, -to hit/dex save, exhaustion, et.al.
I even had the idea that different races might be affected differently by the poison. Perhaps the Grung themselves aren't even aware of the biological difference that the poison has on different species. Perhaps the capybaras, jaguars and snakes they normally hunt, are knocked unconscious when they fail a save, but they've never seen a human, dwarf or elf and the resultant affect may be unique or unexpected.
There's really no limit to the status effects you can choose to afflict your players with to take them captive. Imagine the party's dwarf failing a save after being hit by a dozen darts each round and is now feeling amorous affection for trees and is now acting inebriated and lovingly drunk, dropped his weapon and now swaying to and fro, arm wrapped around a near by sapling and singing a song. Meanwhile the human thief is slumped in her horses saddle horn snoring loudly, and the elven wizard is scratching at his skin from the rash he's developed from the poison.
Perhaps the idea of players rolling a million 20-sideds as saves each round and you just want to move the story along. You can just assume that if the players don't flee or fireball blast the grung, that they only have a certain number of turns before players are overwhelmed by the flurry of darts poisoning them. (say 5 turns for example).
But otherwise, I rule that if a player is hit by a dart, they will suffer a -1 penalty on each successive save they make, starting at DC of 10. So a character that is hit by 4 darts, has to make 4 saves, each with subsequently more difficult DC as the poison starts to build up in their system (DC 10 -0 on first hit, then -1 on the second. then -2 on the third etc.) This increasing difficulty in making the DC roll ensures that over time the players will eventually fail, and reflects the poison building up or taking time to take affect on their heroic constitutions.
