from Wiktionary, Creative Commons Attribution/Share-Alike License
- adj. Used other than as an idiom: see true, blue.
- adj. Steadfastly loyal or faithful, unwavering in loyalty.
- adj. Authentic, undoubtedly true; honest.
Refers to a symbolism of colour, dating to mediaeval times, in which blue represents loyalty, constancy, faithfulness and truth. (Wiktionary)