As a further note on the blue lines in the Cunningham tartan above, I have some more information. The Cunningham tartan first appears in the Vestiarium Scoticum in 1842. The Sobieski Stuarts there describe the lines bordering the white as "blew."

However, in practice, most weavers have done them in black.

Here's a pic of the illustration of the Cunningham tartan from the VS.