Usually Shree Krishna writes in all the Puranas that He is Black (Black Kali-Yuga Avatara), but the devotees perfectly know, that Our Beloved Bhagavan is of very Tender Blue Colour.
By the way, this confusion with colours greatly impairs asuras and Shree Shreemad Jiva Goswami, in "Bhagavad Sandarbha", cites their silly judgments about Shree Krishna, which they make, jumping to the conclusions about Krishna being Nila (Black), Kali (Black) or even Krishna (which also can be translated as "black" from Sanskrit).
Shree Shreemad Shankara-Acharya, in His gloss to the Name NO. 1000 of Bhagavan Shree Vishnu, cites the following verses (page 160 in the English translation):
"Even one salutation heartily rendered to Bhagavan Shree Krishna is equal to ten horse-sacrifices duly performed.
He who has performed ten Ashvamedha sacrifices is again born, but he who salutes Bhagavan Shree Krishna is not. There is no fear to those who bow down before Govinda whose colour resembles that of Atasi flower, who is undecaying and clad in white garment.
The sins committed by man in his past births, in thousands of Kalpas and Pralayas, are immediately destroyed by slightly bowing down his head before the Lord of Three Lokas, the Supreme Lord, Bhagavan Shree Vishnu of uncomparable excellence".
Now, some information about Atasi Flower from http://www.flowersofindia.net/mythology.html
"Atasi अतसी (Linum usitatissimum)
This pretty blue flower is popular in Sanskrit literature for comparing with the complexion of Krishna.
A famous sloka in Krishna's praise goes अतसीपुष्पसंकाशम् हारनूपुरशोभितम् "atasii pushpa sankaasam haara noopura shobhitam".
This flower, along with the blue butterfly-pea flower, Aparaajita, is also popular in worship of Goddess Durga (who is also sometimes considered an "amsha" (अंश) of Krishna).
