The Crow Black Dream:The Myth of the Crow
Ages ago, A snow-white Crow was left by the God Apollo to watch over his love, Coronis. But though Coronis was pregnant with the Sun-God's child, she admitted her passion for another man. The Crow flew off to tell Apollo of Coronis' addmission, but before the Crow could tell him, the Sun-God had already divined Coronis' infidelity.
In the fires of his revenge against Coronis, the Crow was turned black as night. Since that time, the Crows have been spirits of revenge.
Though Coronis died, Apollo managed to save the life of their child...Asclepius. Asclepius grew up and was given a gift by the Goddess Athena; blood from the veins of Medusa the Gorgon. The blood possessed the power to cure the sick, and even to raise the dead. But the power of the Gorgon's blood made Asclepius some powerful enemies, especially Hades, who complained that Asclepius unjustly depleted the population of the underworld. Zeus killed the physician with a thunderbolt...but the Crows kept the Gorgon's blood.
The Crows bring back those who have been wronged, so that they might the wrong things right. And it is said that the Crows shall remain ever black, so long as violence is repaid with violence.
Paraphrazed from "The Myth of the CROW" appearing in "The CROW: Wild Justice" #1, published by Kitchen Sink Comics.
Some versions say Apollo turned the Crow/Raven black in anger and the his twin sister Artemis killed Coronis with arrows. Others say Apollo burned Coronis and the Crow/Raven feathers turned black from the smoke. Crow in this was Corvus, which is Latin for Crow and Raven, and is the name of the constellation. Some people translate the various version leaving Corvus as the name of Apollo's servant, while others call it his Crow/Raven familiar or pet. Coronis' name also means Crow or Raven, and in some versions, she becomes the constellation. In some version, when Artemis kills Coronis, Hermes saves Ascelpius, before taking Coronis to Tartarus, and gave the child to Chiron, the centaur. Chiron was a great healer and trained the child, like he had a number of the gods and heroes. Of the versions I could find online, this was the version I liked the wording of best. It comes from Wild Justice, one of the serious of comics by the author of the graphic novel The Crow, which the movie was based on. The part about the Gorgon's blood comes from The Library by Apollodorus. I can't find any other references to Crows of Ravens saving the vial or being spirits of revenge, so I think that part was added for the comic. The important Raven-related part, then, is the first two paragraphs.
~Muninn's Kiss