A malign infomorph or replicator that uses perversion attacks to increase its own power. Blights can be physical integrated systems like a sentient replicating swarm, or memetic like the Conver Ambi political system. Generally speaking, a blight is something that expands and turns other entities into beings like itself.
Image from Bernd Helfert
Unlike a goo swarm or other disease a blight operates on a transapient level. It may attack entities that are less than transapient, but only to turn them into tools, avatars or servants for itself. Blights are often more obvious than perversions, but still able to field a large array of nasty tricks.