According to Vetron's Guide to Weapons and Armour, the blowgun has a "1" effectiveness rating, and can be loaded with sleep or poison darts.
The blowgun first appeared in Savage Empire, as a weapon outside the "hierarchy" of weapons among the tribes of Eodon; it was most commonly used by Urali thugs. It appears again in the time of Ultima VII, used mostly by scoundrels, and can't be bought. In Ultima VII Part Two, the goblin females of Serpent Isle are very fond of using this weapon with an annoyingly effective poison.
The blowgun, a weapon in use by all the tribes but especially common among the feared Urali thugs, is the 'outcast,' having no place in this hierarchy. The darts alone are far too weak to be a weapon, but when coated with poison they become an instrument of death far too powerful for use under all but the most dire of circumstances.