Muggles' Guide to Harry Potter/Magic/Blood Blisterpod

Overview
A Blood Blisterpod is a purple pod which either causes uncontrollable bleeding, or if bleeding already is occurring, prevents it from stopping.

Extended Description
Ron passes the Quaffle so enthusiastically to Katie Bell at Quidditch practice, that it hits her in the nose and starts a nose bleed. To stop the bleeding, Fred gives her what he believes is the healing end of a Nosebleed Nougat, but it turns out to be a Blood Blisterpod. Katie loses so much blood that she has to be taken to the Hospital Wing.

Analysis
The fact that Fred is carrying a pocket full of Blood Blisterpods would indicate that they are a vital component of some part of the Skiving Snackbox line, most likely the Nosebleed Nougat. Knowing the twins, they are also quite possibly untradeable magical substances under the law, which would explain why we only see them that once.