Blood-stained glass is a quest item used during the quest Salt in the Wound. It is one of the ingredients for making the Anti-mind control serum to stop Mother Mallum. It can be obtained by talking to Bailey, who has finally gone under the control of a sea slug. To do so, players must punch him then ask the slug to prove it shows no pain. The slug will cut himself with a piece of glass and give it to the player.
After the quest, Bailey will complain that his arm has been badly cut and he feels like he has been punched, referencing what the player did in order to obtain blood from him as part of the anti-mind-control serum.