The French term "pure laine" literally meaning pure wool (and often interpreted as true blue;or dyed-in-the-wool) refers to the people having exclusively original ancestry of the French-Canadians. Interestingly enough, the Quebec's flag happens to be blue like the expression.

When a color is "dyed in the wool", the wool itself is dyed before being spun into threads, so the color is least likely to fade or change.