In doing some generalized research for a story, I came across the mention of the term 'blue blood' in reference to those of royal bloodlines. I've heard the term before, but never gave it much thought until now, when the thought clashed with the knowledge that purple is known to be highly recognized in history as the royal color. Was the term blue blood given at a time when blue, not purple, was the primary or most common color for royals? And if not, why do we not refer to a royal as being a purple blood?
Iirc, the term 'blue blood' was used to refer to their translucent skin and the appearance of veins.
The appearance of a tan was more often than not associated with poor labourers, farm hands etc - basically, peasants. There is recorded evidence that a lot of higher end fashion - such as Visards - were worn to prevent this tanning, proving that somebody was wealthy. Naturally, it would make sense in this case to attribute the phrase 'blue blood' to those of nobility.
Similarly, the association of purple with regality simply comes down to the cost of producing purple dye for clothes. Essentially, both the association with the colour purple and the phrase 'blue blood' come down to social status.