Probably the least common of the Border Collie colors or color patterns is the merle. Merle is a dilution of overall body color--black or red--with streaks or splotches of darker color. If the basic color of the dog is black, the merle version will be a soft gray, often referred to as "blue"; if the dog is red, the merle will be a pale red. Because it is uncommon, it is not always recognized correctly. Importantly, the merle pattern occurs on the colored portion of the body, not the white markings. You will often hear the word "merle" or "merling" used to refer to heavy ticking or freckling within the white markings. These patterns are not true merles. Both merles and dogs with a lot of white on their face tend to have blue eyes, adding to the confusion. Because of certain characteristics of the true merle gene, however, it is very important to understand the difference.
Merle is what is known as an epistatic gene; that is, it has an effect on other color genes, not just its own genetic partner. Whatever the basic color genes of the dog--unless he is all white--the merle pattern will cause dilution and splotching. Merle is also, at first glance, dominant. A dog which carries a merle gene will be a merle; only a merle parent can produce a merle puppy. This is, so far, just like the black color, dominant over the red. But merle is different, because it is incompletely dominant. A dog that carries a pair of merle alleles is not just a "pure" or "true-breeding" merle; it has a different color pattern. The "double-merle" dog is mostly white, with a few merle-colored spots. So far, this is OK. Apart from the prejudice against white dogs, we wouldn't particularly care. But the double merle dog has a high probability of serious medical problems. Most of them are completely deaf; many have bad eyes with reduced vision. I think you can see that either of these problems is a serious defect in any dog, but especially in a potentially working dog.
The only way this double merle can be produced is by breeding together two merles; in such a cross, one fourth of the pups (statistically) will be solid colored, half will be simple merles, and one fourth will be double merles. The merle gene is very common, and very popular, in the Australian Shepherd breed (they are sometimes called "those little blue dogs"); it is relatively common among Collies and Shelties as well. In these breeds, where showring demands call for "beautiful" color, double merles are occasionally bred on purpose. The double merle will have exclusively merle puppies, and so has a certain special value as a breeding dog, whatever its individual problems. Of course, the deafness and reduced vision are only passed on in future double merles, and not in the simple merle-patterned offspring.
In Border Collies, where the gene is far less common--even rare--there is no excuse for accidentally breeding a double merle; and there is never a good reason to do it intentionally. In French, the world merle means "blackbird" and "le merle blanc" ("the white blackbird") is an expression meaning "a person or thing that doesn't exist or cannot be found." This should also be the definition of the white merle dog.