One of my favorite pieces of transferware is a loving cup printed with the words: "Who'll ever doubt my Hearts (sic) True Blue." What a lovely gift to give a sweetheart. To me the message also says a lot about the way I love blue printed ware. However, I wanted to know the meaning of the expression "True Blue," so I went to Google.
I learned the phrase was originally Coventry blue, as the dye in the blue cloth made in Coventry, England in the Middle Ages was known to not wash out or fade easily. The phrase was shortened to "true blue." A more complete discussion of the meaning of true blue is found in The Phrase Finder.