Here's the directions:
"Darts are pretty easy to make all you have to do is take a 10 by 10
cm piece of paper and role it into a cone and tape it then you push
it into the end of your blowgun and twist it so it leaves a
indentation on the paper then you cut the paper on the indent, push
a nail (not to big) though the tip so it looks like thistle dart
should fit with no air gaps but should slide easy in barrel. These
are just quick and easy darts and aren't really appropriate for
shooting small game unless the nail is flattened and sharpened into
a broad head.

High Velocity Darts

First make a dart like the one on the blowgun section (above) then
cut some of the cone off the back so the cone is smaller. Now you
need to make a separate device to form a seal behind the dart, make
two cones that fit inside the barrel with no air gaps (not to tight)
then cut the point off one of the cones then push the point of the
other dart in the open point of the cone like this then tape them
together.

To use this you first put the double cone thing in the barrel then
put the high velocity dart cone first down the other end. The idea
of this dart is that when it is fired the dart will be pushed along
the barrel by the double cone thing at the same speed as a normal
dart but unlike a normal dart when it leaves the barrel it has much
less resistance so it will go further and faster."

http://revillusions.bravehost.com/darts.html and
http://revillusions.bravehost.com/Blowguns.html

If anyone makes them - take a picture, and post your results, please.