A blanket party is where a bunch of people grab the blanket and hold it down so that the victim is trapped on the bed. The others involved then beat the snot out of the victim, typically with bars of soap in socks. Frats used to do it for hazing rituals, and it used to be common in the military as a way for soldiers to punish one of their members for poor performance or constantly causing trouble.
In prison it can get more violent, with metal objects or rocks used instead of soap. It’s often used as revenge against snitches. The blanket is also thrown over the victim such that their face is covered, making it impossible for the victim to identify their attackers.
As mentioned above, there is a very well-known scene in the movie Full Metal Jacket where an underperforming soldier is given a blanket party.
