Only one can be carried at any given time, and it cannot be stored in the Chest at the Hunter's Dream.
It is the most powerful Healing Blood in the game. It is also one of the rarest.
Unlike any other type of Blood Vial, or blood of any kind for that matter, it has a pale yellow color to it.
Since there is one found in the Nightmare of Mensis (and the Nightmare of Mensis is likely a physical recreation of a place that already exists), it implies that Iosefka probably conducted her research at the School of Mensis and then relocated.