Dog blood is the blood of a dog.
When going to confront the goddess Veritas, hunter Sam Winchester pulled out a jar of dog blood for him and his brother to dip their knives into as dogs were Veritas' Achilles Heel. Sam admitted that Dean probably didn't want to know how he got the jar of blood.
During a fight with Veritas, Sam was able to kill the goddess by stabbing her in the heart with a knife dipped in dog blood.
While attempting to open Purgatory Castiel is betrayed by Crowley who aligns himself with Raphael. They demand the Purgatory blood Castiel has in order to complete the spell. Castiel manages to trick them and gives them a jar of dog blood instead.
While Sam, Dean, Bobby Singer, are attempting to return the souls Castiel absorbed from Purgatory back to Purgatory the sigil for opening Purgatory drawn in dog blood by Crowley can be seen in the background.
When the Leviathans manage to avoid returning to Purgatory they take over Castiel's body and kill him the process. They then attack Dean and Bobby. The sigil for opening Purgatory drawn in dog blood by Crowley can be seen in the background again.
In order to summon the demon that previously possessed him, Jeffrey needed several ingredients for a ritual to summon the demon. Among these was the heart of a dog. Jeffrey killed his dog and placed its bloodied heart in a bowl for the ritual.