Bluish

Nardole claimed that "some of [his] best friends [were] bluish". (TV: Oxygen)

Behind the scenes
The context in which "bluish" is used by Nardole does not make clear as to whether it simply regards skin colour or is in fact the name of a species. BBC iPlayer subtitles would suggest the former, as the word does not begin with a capital B.

Nardole's claim is an allusion to the popular excuse for denied prejudice, "Some of my best friends are... [insert group or identity here]".

This specific reference tracks to the line "Are you bluish? You don't look bluish", directed from one of the at The Beatles in the 1968 animated film. The original line was itself a pun on the comedy phrase "Funny, you don't look Jewish", which has its own historical connotations.