These are made from the shrunken heads of those killed by Nurgle's favourite plagues. The head of the victim is filled with virulent pus and infected, biting flies and then its orifices are sealed with wax. When the grenade is thrown it splatters across the target area, forming puddles of purulent slime and noisome clouds of buzzing flies.
An enemy unit that has one or more models charged by a model armed with blight grenades suffers a -1 to hit modifier. These effects also count against Chaos models in base contact with models from the enemy unit unless they are followers of Nurgle (ie, Plague Marines, Nurgle Daemons or characters with the Mark of Nurgle). Note that the whole unit is affected, not just the troops fighting the model armed with the blight grenades. There is no additional effect if a unit is attacked by two or more models armed with these grenades.