Blue jets are a form of lightning which moves up from a thundercloud, rather than down to the ground or across to other clouds. There have been occasional reports of sightings but it was not until 1990 that actual videos were taken by one John R. Winckler of the University of Minnesota to confirm the phenomenon.
One reason for the obscurity of blue jets, red sprites and other forms of high-atmosphere lightning is that they are simply hard to observe. These effects take place above the clouds and the emitted light is very brief and fairly dim, so it must usually be viewed from above the clouds and against a dark background. Winckler's fortuitous discovery has since spurred entire campaigns of investigation into high-atmosphere lightning.
Apart from being impressive to look at, high-atmosphere lightning is also very interesting to scientists for a number of reasons. One is that the upward flashes reach all the way to the ionosphere; another is that the upward lightning seems to occur at the same time as "normal" flashes of lightning to the ground or other clouds. This leads scientists to believe that there is a kind of atmospheric circuit being closed by these discharges. Despite recent progress in the field, speculation into the physical underpinnings of the phenomenon goes on.
References
- http://elf.gi.alaska.edu/sprites.html
- http://www.sciam.com/0897issue/0897mende.html