Why is Krishna blue?

from Srimad-Bhagavatam

The Supreme Personality of Godhead has a cheerful, lotus-like countenance with ruddy eyes like the interior of a lotus and a swarthy body like the petals of a blue lotus. He bears a conch, discus and mace in three of His hands.

"The color of the Personality of Godhead, Krishna, is described here as nilotpala-dala, meaning that it is like that of a lotus flower with petals tinted blue and white. People always ask why Krishna is blue. The color of the Lord has not been imagined by an artist. It is described in authoritative scripture.
In the Brahma-samhita also, the color of Krishna's body is compared to that of a bluish cloud. The color of the Lord is not poetical imagination. There are authoritative descriptions in the Brahma-samhita, Srimad-Bhagavatam, Bhagavad-gita and many of the Puranas of the Lord's body, His weapons and all other paraphernalia. The Lord's appearance is described here as padma-garbharekshanam. His eyes resemble the inside of a lotus flower, and in His four hands He holds the four symbols: conchshell, discus, mace and lotus."

Srimad-Bhagavatam 3.28.13, Purport