That was a long time ago. A sage sent a crow to search for the nectar but also warned the crow that only to find out about the nectar is not to drink it, otherwise you will have to bear sin. The crows agreed and then the white crows left the sage. Earlier all crows were white colored.
After a year of hard work, the crows finally came to know about nectar. He could not stop the craving to drink it and drank it while the sage strictly forbade him to drink it. So he broke his promise to the sage by doing this. After drinking, he repented and he came back and told the whole thing to the sage, when the sage heard this, he got angry and cursed the crows and said because you have destroyed the sanctity of the nectar with your unholy beak, hence the whole after today Mankind will hate you and only you will be seen among the most hated birds. Like an inauspicious bird, the entire human race will always condemn you.
Since you have drunk nectar, there will be no natural death and no disease and you will not get old age either.
On the sixteen days of the month of Bhadrapada, you will be respected as a symbol of ancestors. Saying so that you will die accidentally, the sage dipped it in the black water of his kamandal. Since then, the crow has turned black. The crows have been black since then.