I came across some websites that mention that Goddess Neela Saraswathi and Nila Devi, the third wife of Lord Vishnu, could be the same. The websites are here and here. Is this fact mentioned in any scriptures like Devi Bhagavatham?
Nila Saraswati is another name/form of Goddess Tara, the second Dashamahavidya. So, she isn't the wife of Lord Vishnu.
2.Munda mala vibhooshithe neela rupinee ,
Yeka jata neela Saraswathi nama Vikhyathe, Tara devi namosthuthe.
Salutations to Goddess Tara , Who wears the necklaces of skulls, Who is blue in colour , Who is well known as Yeka Jata and Neela Saraswathi.
Source - Dashamahavidya Stotram.
Goddess Tara has various Murtis (forms) as per Tantras and one among which is the Nila Saraswati form.
TArA chogrA mahogrAcha vajrA nilA-saraswati |
KAmeswari bhadrakAli ityashtau tArini smritA ||
This verse from Nila Tantram mentions various 8 forms of the Goddess like Ugra, Maha-Ugra, VajrA etc. Nilasaraswati is one among them.
Similarly, the Tara-Rahasya states:
AdyA chaikajatA proktA dvitiyA chogra-tArakA |
TritiyA nilavAni syAd bhoga-moksha-pradA matA ||
This verse states that the 1st form of the Goddess is called Ekajata, the 2nd form is Ugratara and the 3rd form is nilavAni which is the same as Nilasaraswati.