See page 88 in WHFB 3rd Edition for the full rules, the D6 Strength is a special rule for the blowpipe. As it is technically a type of poisoned attack it would count as a S1 attack against Skeletons and other immune to poison models. A blowpipe can be poisoned and get the +1 Strength for normal poisoned attacks - such is the case for the Slann Venom Tribes and the scout pygmy units. Don't forget that shots over 6" range will be at -1 to hit, moving would be an additional -1 making it fairly difficult to do much damage with them given the Slann BS of 2. The BS3 Venom tribes with poisoned attacks are just about the only viable missile unit in the Warhammer Armies Slann list and they are not too bad in hand to hand with their poisoned attacks.