Ref: http://physicsweb.org/article/news/7/11/12

Physicists at Glasgow University in the UK
have observed for the first time the hidden
colours that were recently predicted to
exist in "dark light" (J Leach and M Padgett
2003 New J. Phys. 5 154). Dark light is
produced when two or more beams of light
interfere destructively and cancel each
other out to form a spot that appears
completely black to the human eye.

The phenomenon of dark light is closely
associated with regions of space known as phase
dislocations or singularities: these are regions
where the phase of the light is not defined and can
have any value between zero and 2?. If the phase
singularity is a line that runs parallel to the axis of
the beam it is known as an "optical vortex".

See: http://physicsweb.org/article/news/7/11/12