The expression 'true blue' derives from the blue cloth that was made at Coventry, England in the late middle ages. The town's dyers had a reputation for producing cloth that didn't fade with washing. The color remained fast and true!
John Ray wrote in the first edition of a Compleat Collection of English Proverbs in 1670:
"Coventry had formerly the reputation for dying of blues; insomuch that true blue became a Proverb to signifie one that was always the same and like himself."
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