Lamb blood is the blood of a lamb.
History
In the Fantasy Universe of What Is And What Should Never Be, Dean arms himself with a silver knife from his mother's silverware and is able to get a jar of lamb blood to use on the djinn he is hunting. The Fantasy Universe Sam is horrified to see the blood and to learn of Dean's beliefs about the djinn. Upon realizing the truth, Dean wakes himself up by stabbing himself with the knife he has dipped in lamb blood in anticipation of killing the djinn.
In the real world, Dean is found by Sam who is armed with a knife dipped in lamb blood. During the fight that follows, Dean kills the djinn with Sam's knife.
In Exile on Main Street, while hunting Brigitta and her brothers, the Winchesters arm themselves with knives dipped in lamb blood. While they never get to use them, Samuel Campbell kills one of the djinn with a knife of his own and threatens Brigitta with it as a distraction so that Christian can capture her.
Later, the angel Balthazar, while on the run from Raphael's followers, used the blood of a lamb as an ingredient in the spell to send Sam and Dean to an alternate universe.
In Pac-Man Fever, after realizing that they are dealing with a bastard offshoot of a djinn, the Winchesters once more arm themselves with silver knives dipped in lamb blood. Dean is able to kill Jennifer O'Brien in this way and Sam her son.
In All Along the Watchtower, the birth of the nephilim Jack opens a portal to another universe. Sam, Dean, Crowley, and Castiel attempt to close the portal while also trapping Lucifer there at the same time. Crowley knew a spell that could close the rift. One of the ingredients needed to perform the spell was lamb blood. As Dean distracted Lucifer, Sam and Crowley prepared the ritual before Crowley killed himself with an angel blade to complete it. The ritual took a few minutes to fully close the rift, resulting in the death of Castiel and Mary Winchester getting trapped with Lucifer when the rift closed.
In Good Intentions, Donatello asks for virgin lamb's blood to supposedly make a spell to open a rift to other universes. However, it was later revealed that Donatello was lying.