Late Middle English (in the Latin sense): from Latin candor 'whiteness'. The current sense dates from the mid 18th century; the development of the senses paralleled that of candid.

• candid from mid 17th century:

  ‘The stones came candid forth, the hue of innocence’, wrote the poet John Dryden around 1700. He was using the word candid in its original meaning ‘white’, from Latin candidus. Over time the English word developed the senses ‘pure and innocent’, ‘unbiased’, and ‘free from malice’, before finally settling on the meaning ‘frank’. Candour (Late Middle English) has a similar history, its meaning developing from ‘whiteness’ to the current ‘openness and honesty in expression’. See also album. These days someone running for office needs to be ‘whiter than white’. So did the candidates in Roman times, since the word candidate, is also based on candidus. A candidatus was a white-robed person, as candidates for office were traditionally required to wear a pure white toga or robe, meant to reflect their unstained character.