I thought I had read on Matt Newsome's blog that solid black kilts had been adopted as a dress/symbol for the people of Cornwall.
If I remember correctly, some Cornish were using a solid back kilt, but now there a number of Cornish tartans:

http://www.cornish-tartans.co.uk/

Notice the St. Piran's Cross, a white cross on a black field, is prominent -- St. Piran is the patron saint of Cornwall and tin miners.

Cheers,
Todd