Some people out in
lightning storms say they have seen
black lightning. This later developed into something of an urban
myth. Eventually some scientists went out to record this, but it turned out to be nothing more than the after-image or regular lightning, left on the human
retina.
Want to see black lightning for yourself? Find a lightning storm, and look at a lightning bolt (The brighter, the better). Right after seeing one, move your head or your eyes to the side. The after-image will make it look as if a negative version of the lightning has struck again in someplace else. That's it.