The blood sac (not to be confused with the blood pack) consists of the severed abdomen of a bloodbug, filled to bursting with blood drained from the bloodbug's latest prey. It is a source of antiseptic and fiberglass, possibly as a result of the bloodbug's mutant biochemistry mixed with the blood in its gut.
Blood sac (1)
Although they cannot be found in the world, blood sacs are rarely dropped by super mutants. They can also be found in meat bags.