Spoiling a stone of the opponent's, by leaving it too close to a strong position of yours at the end of a tactical sequence.
(Literally blight is a kind of fungus or rot that affects farmers' crops such as grain or vegetables.)
This position from BQM64 explains a common technique combining the dropping back and blighting concepts.
here is no good as a way to cut. After Black shouldn't continue.
If Black persists, this sequence already leaves blighted. Black has caused problems for himself.
In this common joseki, is left next to strong black stones, and so is blighted. Losing this stone is the price white pays for making territory on the top side. In this case, is not a mistake. This is joseki.
More information here.
 If you blight an opponent's stone, are you a blighter? ;-)