The acid blob, b, is an amoeboid monster in NetHack. They can be generated at will by reading scrolls of create monster while confused. Their corpses, in addition to being acidic (therefore curing stoning when eaten), can be sacrificed at any time after being killed (and are unique in being the only monster for which this is true). Of course, unlike the corpses of lichens and lizards, they may rot away before they can be sacrificed.
Acid blobs have no offensive properties and pose no direct threats; however, they pose a dastardly trap for the inexperienced player. Attacking them may prompt a reactive splash of acid, which will cause HP loss (unless acid resistant) and may corrode susceptible weapons and armor. Eating their corpse will give the message "you have a severe case of stomach acid" and cause further HP loss unless you have acid resistance. Additionally, their viscous nature allows them to flow under closed doors, making them a persistent (if slow) pursuer and possibly causing the player to run into them by accident. They may be peaceful if you are the same alignment.
Acid blobs are candidates for filling up a dungeon level so that other monsters cannot revive; they are harmless and can be generated rapidly if you have enough scrolls of create monster. This is one way to take care of the Riders on the Astral Plane.