Every breed has a “Standard” which describes the ideal specimen of the breed. The Standard describes the “perfect” specimen of each breed – its size, colour, proportions, the purpose (work) of the breed, gait, etc as well as acceptable colours. These standards may vary slightly from country to country, especially with changes to cropping and docking laws in Europe and Australia.
Here are links to the Cardigan Welsh Corgi Standard in different countries. The Cardigan Standard DOES include provision for Blue Merle
U.K. (Country of Origin)