As I discuss in this answer, the Pancharatra Agamas are the foundational texts of Vaishnavism. Similarly, the foundational texts of Shaivism are known as the Shaiva Agamas, aka the Shaiva Tantras. One of the Shaiva Agamas is known as the Kirana Agama, and it takes the form of a dialogue between Shiva and Vishnu's bird Garuda.
In this excerpt from the Kirana Agama, Garuda says a hymn of praise to Shiva, and in particular he says this:
Victory to the Lord who quenched the virility of the deadly poison (emitted by the snake Vasuki at the time of the churning of the milky ocean) that blackened the body of Vishnu (who was then white in complexion); victory to the Lord who sustained the divine river Ganga that rushed forth from the heaven ... full of whirls and great conceit.
My question is, what scriptures describe Vishnu's body being blackened by Vasuki during the Churning of the Ocean?
This chapter of the Srimad Bhagavatam does describe Vasuki blackening the faces of the Devas, at least before Vishnu sends down rain to remedy the situation:
Vāsuki had thousands of eyes and mouths. From his mouths he breathed smoke and blazing fire, which affected the demons, headed by Pauloma, Kāleya, Bali and Ilvala. Thus the demons, who appeared like sarala trees burned by a forest fire, gradually became powerless. Because the demigods were also affected by the blazing breath of Vāsuki, their bodily lusters diminished, and their garments, garlands, weapons and faces were blackened by smoke. However, by the grace of the Supreme Personality of Godhead, clouds appeared on the sea, pouring torrents of rain, and breezes blew, carrying particles of water from the sea waves, to give the demigods relief.
And Vishnu's incarnation Ajita, the Manvantara Avatara of the Chakshusha Manvantara, was one of the Devas who participated in the churning of the Ocean. But are there any scriptures that specifically say that he was blackened? And is there any scriptural basis for the translator's claim that he was originally white?
On a side note, I suppose if Vishnu was originally white, that would lend support to the theory that the "shuklam bharadharam" shloka chanted before the Vishnu Sahasranamam is addressed to Vishnu rather than Ganesha or Vishvaksena.