I posted about this earlier in the GR, but unfortunately I can't access my old post. :-( Like you, I always understood "candid" to mean "sincere", as per the modern meaning, but according to a note in the Oxford World's Classics edition of P&P that I have, the word was defined differently in Dr. Johnson's dictionary:

free from malice; not desirous to find faults

and quotes someone saying "the ingenious lady in "The School for Scandal" (1777) had not Candour but an Affectation of it".

However, now that I go to look up "candid" in the online 1812 edition of Dr. Johnson's Dictionary, I see that the definitions are given as 1) white and 2) fair; open; ingenuous, and "sweetness of temper; purity of mind; ingenuity" for "candour", so I don't know which edition the OWC got their definition from! :-(